Tag Archives: R-english

Beta kernel and transformed kernel

This Thursday I will give a talk at Laval University, on “Beta kernel and transformed kernel : applications to copula density estimation and quantile estimation“. This time, I will talk at the department of Mathematics and Statistics (13:30 at the pavillon Adrien-Pouliot). “Because copulas have bounded support (the unit square in dimension 2), standard kernel based estimators of densities are (multiplicatively) biased on borders and in corners of the support. Two techniques can be used to avoid that underestimation: Beta kernels and Transformed kernel. We will describe and discuss those two techniques in the first part of the talk. Then, we will see that it is possible to combine those two techniques to get nice estimator of several quantities (e.g. quantiles): transform the data to get on the unit interval – using a transformed kernel – then estimate the (transformed) quantile on [0,1] using a beta kernel, then get back on the initial support. As we will see on simulations, that technique can be better than standard quantile estimators, especially when data are heavy tailed.” Slides can be downloaded here.

  • kernel based density estimation

Kernel based estimation are a popular (and natural) technique to estimate densities.  It is simply and extension of the moving histogram:

so we count how many observations are a the neighborhood of the point where we want to estimate the density of the distribution. Then it is natural so consider a smoothing function, i.e. instead of a step function (either observations are close enough, or not), it is possible to give weights to observations, which will be a decreasing function of the distance,

With a smooth kernel, we have a smooth estimation of the density


Then it is possible to play on the bandwidth, either to get a more accurate estimation of the density, but not that smooth (small bias but large variance),

or a smoother one (large bias, but small variance),

In R, it is simply

> X=rnorm(100)
> (D=density(X))
	density.default(x = X)
Data: X (100 obs.);	Bandwidth 'bw' = 0.3548
       x                   y            
 Min.   :-3.910799   Min.   :0.0001265  
 1st Qu.:-1.959098   1st Qu.:0.0108900  
 Median :-0.007397   Median :0.0513358  
 Mean   :-0.007397   Mean   :0.1279645  
 3rd Qu.: 1.944303   3rd Qu.:0.2641952  
 Max.   : 3.896004   Max.   :0.3828215  
> plot(D$x,D$y)
  • Beta kernel

The idea of Beta kernel is to consider kernels having support [0,1]. In the univariate case,


where http://freakonometrics.blog.free.fr/public/perso3/kernel-f-07.gif is the density of a Beta distribution, i.e.

http://freakonometrics.blog.free.fr<br />

For additional material, I have uploaded some R code to fit copula densities using beta kernels,

beta.kernel.copula.surface = function (u,v,bx,by,p) {
s = seq(1/p, len=(p-1), by=1/p)
mat = matrix(0,nrow = p-1, ncol = p-1)
for (i in 1:(p-1)) {
a = s[i]
for (j in 1:(p-1)) {
b = s[j]
mat[i,j] = sum(dbeta(a,u/bx,(1-u)/bx) *
dbeta(b,v/by,(1-v)/by)) / length(u)
} }
return(data.matrix(mat)) }

Then we can used it to see what we get on a simulated sample

COPULA = frankCopula(param=5, dim = 2)
X = rcopula(n=1000,COPULA)
p0 = 26
Z= beta.kernel.copula.surface(X[,1],X[,2],bx=.01,by=.01,p=p0)
u = seq(1/p0, len=(p0-1), by=1/p0)

(yes, the surface is changing… to illustrate the impact of the bandwidth on the estimation).

  • transformed kernel estimation

I the talk, I will also mention the transformed Kernel estimate, as introduced in the book on L1 density estimation by Luc Devroye and Laszlo Györfi (the book can be downloaded here). I probably spend a few minutes on the original chapter, in order to provide another application of that techniques (not only to estimate copula densities, but here to estimate quantiles of heavy tailed distribution). In the univariate case, the R code is the following (here I consider two transformation, the quantile function of the Gaussian distribution, and the quantile function of the Student distribution with 3 degrees of freedom),

transfN = function(x){
transfT = function(x){

The density estimation is the following,

(the red dotted line is the true density, since we work on a simulated sample). Now, let us get back on the initial chapter,

In the book, this is introduced as follows,

The original idea we add it to use this kernel based estimator for copulas, i.e. since we can estimate densities in high dimension with unbounded support, using


the idea is to transform marginal observations,


and to use the fact that the associated copula density can be written


to derive an intuitive estimator for the copula density


An important issue is how do we choose the transformation

And Luc Devroye and Laszlo Györfi mention that this can be used to deal with extremes.

well, extremes are introduced through bumps (which is not the way I would have been dealing with extremes)

and note that several results can be derived on those bumps,


Then, there is an interesting discussion about estimating the optimal transformation

and I will prove that this can be an extremely interesting idea, for instance to estimate quantiles of heavy tailed distribution, if we use also the beta kernel estimator on the unit interval. This idea was developed in a paper with Abder Oulidi, online here.

Remark: actually, in the book, an additional reference is mentioned,

but I have never been able to find a copy… if anyone has one, I’d be glad to read it…

Time horizon in forecasting, and rules of thumb

I recently received an email about forecasting and rules of thumb. “Dans la profession […] se transmet une règle empirique qui voudrait que l’on prenne un historique du double de l’horizon de prévision : 20 ans de données pour une prévision à 10 ans, etc… Je souhaite savoir si cette règle n’aurait pas, par hasard, un fondement théorique quitte à ce que le rapport ne soit pas de 2 pour 1, mais de 3 pour 1 ou de 1 pour 1 par exemple.” To summarize briefly, the rule is to consider a 2-1 ratio for the period of observation vs. forecast horizon. And the interesting question is if there are justifications for such a rule…

At first, I remembered a rules of thumb, from the book by Box and Jenkins, which states that it is meaningless to look at autocorrelations when lags exceed the sample size over 6. So with 12 years of data, autocorrelations with a lag higher than two years are useless. But it is not what is mentioned here. So I looked at some dataset, and some standard time series models.

  • It depends on the series

It might obvious… but if it is the case, it means that it will be difficult to have a general rule of thumb. Consider e.g. the number of airline passengers,

X = AirPassengers
ETS = ets(X)

or some sales in a big store,

or car casualties in France, or the temperature in Nottingham Castle,

or the water level at Lack Hurron, or the flow of the Nile river,

or see also here for forecasting techniques in demography. Actually, in the case of life insurance, actuaries have to forecast future demography, i.e. try to assess death rates of those who currently purchase retirement contracts, who might be 20 years old. So they have to forecast death rate until 2100, say. One the one hand, it sounds difficult to make forecast over a century (it is already difficult for climate, I guess it is even more complex for human life). On the other hand, a 2-1 ratio means that we have to use data from 1800… Here again, it is difficult to justify that mortality in the 1850 could be interesting to say anything about mortality in 2050. So I guess it will be difficult to justify the use of general rules of thumb….

  • It depends on the model

Consider the following (simulated) series. Several models can be fitted. And the shape on the forecast (and the forecast error) will depend on the model considered. The benchmark can be the model without any dynamics, i.e. we assume that observations are i.i.d. Or more classically, assume that it is simple a white noise, i.e. an i.i.d centered process. Then the forecast is the following,

With that kind of assumption, we see that the 2-1 ratio is useless since we can get forecasts up to any horizon…. But that does not seem very robust. For instance, if we consider exponential smoothing techniques, we can obtain

Which is rather different. And with the 2-1 ratio, obviously, there is a lot of uncertainty at the end ! It would be even worst if we assume that we look at a random walk. Because actually a dozen models – at least – can be considered, from ARIMA, seasonal ARIMA, Holt Winters, Exponential Smoothing, etc…


So I do not see any theoretical justification of that rule of thumb. Obviously, the maximum horizon can not be extremely far away if the series is non-stationary, with a very irregular pattern, and with a lot of noise… So we’re back at the beginning. If anyone is willing to share his or her experience, comments are open.

Circular or spherical data, and density estimation

I few years ago, while I was working on kernel based density estimation on compact support distribution (like copulas) I went through a series of papers on circular distributions. By that time, I thought it was something for mathematicians working on weird spaces…. but during the past weeks, I saw several potential applications of those estimators.

  • circular data density estimation

Consider the density of an angle say, i.e. a function https://f-origin.hypotheses.org/wp-content/blogs.dir/253/files/2015/12/circ-01.gif such that


with a circular relationship, i.e. https://f-origin.hypotheses.org/wp-content/blogs.dir/253/files/2015/12/circ-03.gif. It can be seen as an invariance by rotation.
von Mises proposed a parametric model in 1918 (see here or there), assuming that


where https://f-origin.hypotheses.org/wp-content/blogs.dir/253/files/2015/12/circ-05.gif is Bessel modified function of order 1,


(which is simply a normalization parameter). There are two parameters here, https://f-origin.hypotheses.org/wp-content/blogs.dir/253/files/2015/12/circ-07.gif (some concentration parameter) and mu a direction.
From a series of observed angleshttps://f-origin.hypotheses.org/wp-content/blogs.dir/253/files/2015/12/circ-08.gif, the maximum likelihood estimator for kappa is solution of






and where https://f-origin.hypotheses.org/wp-content/blogs.dir/253/files/2015/12/circ-12.gif, where those functions are modified Bessel functions. Well, that estimator is biased, but it is possible to improve it (see here or there). This can be done easily in R (actually Jeff Gill – here – used that package in several applications). But I am not a big fan of that technique….

  • density estimation for hours on simulated data

A nice application can be on the estimation of the daily density of a temporal events (e.g. phone calls as we’ll see later on, or email arrival time). Let https://f-origin.hypotheses.org/wp-content/blogs.dir/253/files/2015/12/circ-13.gif is the time (in hours) for the https://f-origin.hypotheses.org/wp-content/blogs.dir/253/files/2015/12/circ-14.gifth observation (the https://f-origin.hypotheses.org/wp-content/blogs.dir/253/files/2015/12/circ-14.gifth phone call received). Then set


The time is now seen as an angle. It is possible to consider the equivalent of an histogram,

plot(Ht, stack=FALSE, shrink=1.3, cex=1.03,
points(Ht, rotation = "clock", zero =c(rad(90)),
col = "1", cex=1.03, stack=TRUE )


or a kernel based estimation of the density (the gray line on the right).

circ.dens = density(Ht+3*pi/2,bw=20)
plot(Ht, stack=TRUE, shrink=.35, cex=0, sep=0.0,
xlim=c(-2,2),ylim=c(-2,2), ticks=TRUE, tcl=.075)
lines(circ.dens, col="darkgrey", lwd=3)
text(0,0.8,"24", cex=2); text(0,-0.8,"12",cex=2);
text(0.8,0,"6",cex=2); text(-0.8,0,"18",cex=2)

The code looks rather simple. But I am not very comfortable using codes that I do not completely understand. So I did my own. The first step was to get a graph similar to the one we have on the right, except that I prefer my own kernel based estimator. The idea is that instead of estimating the density on https://f-origin.hypotheses.org/wp-content/blogs.dir/253/files/2015/12/Xi.gif, we estimate it on the sample https://f-origin.hypotheses.org/wp-content/blogs.dir/253/files/2015/12/circular-density-3.gif. Then we multiply by 3 to get the density only on https://f-origin.hypotheses.org/wp-content/blogs.dir/253/files/2015/12/0-24.gif. For the bandwidth, I took the same as the one that we would have taken on https://f-origin.hypotheses.org/wp-content/blogs.dir/253/files/2015/12/Xi.gif

The code is simply the following


plot(cos(O),-sin(O),xlim=c(-2,2),ylim=c(-2,2), type="l",axes=FALSE,xlab="",ylab="") for(i in pi/12*(0:12)){ abline(a=0,b=tan(i),lty=1,col="light yellow")} segments(.9*cos(O12),.9*sin(O12),1.1*cos(O12),1.1*sin(O12)) lines(Dd*cos(Od),-Dd*sin(Od),col="red",lwd=1.5) text(.7,0,"6"); text(-.7,0,"18") text(0,-.7,"12"); text(0,.7,"24") R=1/24/max(d$y)/3+1 lines(R*cos(O),R*sin(O),lty=2)

Note that it is possible to stress more (visually) on hours having few phone calls, or a lot (compared with an homogeneous Poisson process), e.g.

for(i in pi/12*(0:12)){
abline(a=0,b=tan(i),lty=1,col="light yellow")}
segments(2*cos(O12),2*sin(O12),1.1*cos(O12),1.1*sin(O12), col="light grey")

We get here those two graphs,

To be honest, I do not really like that representation – even if it looks nice. If we compare that circular representation to a more classical one (from 0:00 till 23:59 one the graph on the left, below), I do have a problem to interpret the areas in blue and pink.

density of wind direction

On the left, we compare two densities, so the area in pink is the same as the area in blue. But here, it is no longer the case: the area in pink is always larger to the one in blue. So it might help so see when we have a difference, but there is a scaling issue that we cannot discuss further… But less us see if we can use that estimation technique to several problems.

A standard application when studying angles is wind direction. For instance, in Montréal, it is possible to find hourly observations, starting in 1974 (we just need a R robot to pick up the information, but I’ll tell more about that in another post, someday). Here, we have directly an angle. So we can use a code rather similar to the one used above to estimate the distribution of wind direction in Montréal.

density of 911 phone calls

Note that our estimate is consistent with several graphs that can be found on meteorological websites (e.g. the one above on the right, that was found here).

In a recent post (here) I wanted to check about the “midnight crime” myth, using hours of 911 phone calls in Montréal.

That was for all phone calls. But if we look more specifically, for burglaries, we have the distribution on the left, and for conflicts the one on the right

We do clearly observe that gun shots occur a bit before midnight. See also here for another study, but this time in NYC (thanks @PAC for the link).while for gun shots, we have the distribution on the left, and for “troubles” (basically people making too much noisy in parties) or “noise” the one on the right

  • density of earth temperatures, or earthquakes

Of course it is also possible to work in higher dimension. Before, we went from densities on https://f-origin.hypotheses.org/wp-content/blogs.dir/253/files/2015/12/circ-16.gif to densities on the unit circle https://f-origin.hypotheses.org/wp-content/blogs.dir/253/files/2015/12/circ-18.gif. But similarly, it is possible to go from https://f-origin.hypotheses.org/wp-content/blogs.dir/253/files/2015/12/circ-17.gif to the unit sphere https://f-origin.hypotheses.org/wp-content/blogs.dir/253/files/2015/12/circ-19.gif. A nice application being global climate studies,

The idea being that point on the left above are extremely close to the one on the right. An application can be e.g. on earthquakes occurrence. Data can be found here.

Hpi1 = Hpi(x = X)
DX=kde(x = X, H = Hpi1)
cbind(X[,1]+360,X[,2]+180),cbind(X[,1]-360,X[,2]+180), cbind(X[,1],X[,2]-180),cbind(X[,1]+360, X[,2]-180),cbind(X[,1]-360,X[,2]-180)) DY=kde(x = Y, H = Hpi1) library(maps) plot (DY,add=TRUE,col="purple")

Without any correction, we get the red level curves. The pink one integrates correction.

Want to say one thing and the exact oppositive with strong confidence ?

No need to do politics. Just take a statistical course. And I do not talk about misinterpretation of statistics, but I talk about the mathematical foundations of statistical tests.
Consider the following parametric test, with a one-dimensional parameter: http://freakonometrics.blog.free.fr/public/perso2/test-lies-01.gif versus http://freakonometrics.blog.free.fr/public/perso2/test-lies-02.gif, for some (fixed) http://freakonometrics.blog.free.fr/public/perso2/test-lies-03.gif. A standard way of doing such a test is to consider an rejection region http://freakonometrics.blog.free.fr/public/perso2/test-lies-05.gif. The test works as follows: consider a sample http://freakonometrics.blog.free.fr/public/perso2/test-lies-06.gif,

  • if http://freakonometrics.blog.free.fr/public/perso2/test-lies-07.gif, then we accept http://freakonometrics.blog.free.fr/public/perso2/test-H0.gif
  • if http://freakonometrics.blog.free.fr/public/perso2/test-lies-09.gif, the we reject http://freakonometrics.blog.free.fr/public/perso2/test-H0.gif

For instance, consider the case of a Bernoulli sample, with probability http://freakonometrics.blog.free.fr/public/perso2/test-lies-62.gif. The standard idea is to define


The rejection region is then based on statistic http://freakonometrics.blog.free.fr/public/perso2/test-lies-210.gif,

  • if http://freakonometrics.blog.free.fr/public/perso2/test-lies-25.gif, then we accept http://freakonometrics.blog.free.fr/public/perso2/test-H0.gif
  • if http://freakonometrics.blog.free.fr/public/perso2/test-lies-22.gif, the we reject http://freakonometrics.blog.free.fr/public/perso2/test-H0.gif

where threshold http://freakonometrics.blog.free.fr/public/perso2/test-lies-26.gif is taken so that the probability to make a first type error is http://freakonometrics.blog.free.fr/public/perso2/test-lies-28.gif(say 5%) using the Gaussian approximation for z. Here


Thus, the acceptation region is then the green area below, while the rejection region is the red one, for http://freakonometrics.blog.free.fr/public/perso2/test-lies-210.gif.

Consider now the exact opposite test (with the same http://freakonometrics.blog.free.fr/public/perso2/test-lies-03.gif), http://freakonometrics.blog.free.fr/public/perso2/test-lies-51.gifversus http://freakonometrics.blog.free.fr/public/perso2/test-lies-52.gif. Here, we use the same statistics, and the test is

  • if http://freakonometrics.blog.free.fr/public/perso2/test-lies-22.gif, then we accept http://freakonometrics.blog.free.fr/public/perso2/test-H0.gif
  • if http://freakonometrics.blog.free.fr/public/perso2/test-lies-25.gif, the we reject http://freakonometrics.blog.free.fr/public/perso2/test-H0.gif

where now


Thus, now, the acceptation region is then the green area below, while the rejection region is the red one.

So if we summarize what we just said,

  • in the region on the left below, both test agree that http://freakonometrics.blog.free.fr/public/perso2/test-lies-55.gif
  • in the region on the right below, both test agree that http://freakonometrics.blog.free.fr/public/perso2/test-lies-57.gif
  • and in the region in blue, in the middle, the two tests disagree (one claims that http://freakonometrics.blog.free.fr/public/perso2/test-lies-55.gif, and the other one that http://freakonometrics.blog.free.fr/public/perso2/test-lies-57.gif)

Here is the evolution of the region as a function of http://freakonometrics.blog.free.fr/public/perso2/test-lies-56.gif (the size of the sample) when the sample frequency is 20%. With a small sample size, we can hardly say anything.

polygon(c(n,rev(n)),c(x1,rev(x2)),col="light blue",border=NA)

One might say that those bounds are based on a Gaussian approximation which is not correct when http://freakonometrics.blog.free.fr/public/perso2/test-lies-56.gif is too small. So we can compute exact bounds,

and we get

This is what we can observe if we use R statistical procedures, either the asymptotic one,

> prop.test(2,10,.5,alternative="less")
1-sample proportions test with continuity correction
data:  2 out of 10, null probability 0.5
X-squared = 2.5, df = 1, p-value = 0.05692
alternative hypothesis: true p is less than 0.5
95 percent confidence interval:
0.0000000 0.5100219
sample estimates:
> prop.test(2,10,.5,alternative="greater")
1-sample proportions test with continuity correction
data:  2 out of 10, null probability 0.5
X-squared = 2.5, df = 1, p-value = 0.943
alternative hypothesis: true p is greater than 0.5
95 percent confidence interval:
0.04368507 1.00000000
sample estimates:

or a more accurate one

> binom.test(2,10,.5,alternative="less")
Exact binomial test
data:  2 and 10
number of successes = 2, number of trials = 10, p-value = 0.05469
alternative hypothesis: true probability of success is less than 0.5
95 percent confidence interval:
0.0000000 0.5069013
sample estimates:
probability of success
> binom.test(2,10,.5,alternative="greater")
Exact binomial test
data:  2 and 10
number of successes = 2, number of trials = 10, p-value = 0.9893
alternative hypothesis: true probability of success is greater than 0.5
95 percent confidence interval:
0.03677144 1.00000000
sample estimates:
probability of success

Here, when the sample frequency is 20% and http://freakonometrics.blog.free.fr/public/perso2/test-lies-56.gif is equal to 10, we accept at the same time that theta is higher than 50% and lower than 50%.
And obviously it is not only a theoretical problem: it has obviously some strong implications. This morning, a good friend mentioned a post published some months ago, online here, about discrimination, and the lack of women with academic positions in mathematics, in France. As claimed by the author of the post“A Paris VI, meilleure université française selon son président, sur 11 postes de maitres de conférences, 5 filles classées premières. Il y a donc des filles excellentes ? A Toulouse, sur 4 postes, 2 filles premières. Parité parfaite. Mais à côté de cela, Bordeaux, 4 postes, 0 fille première. Littoral, 3 postes, 0 fille, Nice, 5 postes, 0 fille, Rennes, 7 postes, 0 fille…”.
Consider the latter one: in Rennes, out of 7 people hired last year, no woman. So in some sense, it looks obvious that there is some kind of discrimination ! Zero out of seven ! Well, if we consider the fact that around 30% of PhD thesis in mathematics were defended by women those years, we can also try to see is there if no “positive discrimination“, i.e. test http://freakonometrics.blog.free.fr/public/perso2/test-lies-60.gif where theta is the probability to hire a woman (just to be a little bit provocative).

> prop.test(0,7,.3,alternative="less")
1-sample proportions test with continuity correction
data:  0 out of 7, null probability 0.3
X-squared = 1.7415, df = 1, p-value = 0.09347
alternative hypothesis: true p is less than 0.3
95 percent confidence interval:
0.0000000 0.3719021
sample estimates:
Warning message:
In prop.test(0, 7, 0.3, alternative = "less") :
Chi-squared approximation may be incorrect
> binom.test(0,7,.3,alternative="less")
Exact binomial test
data:  0 and 7
number of successes = 0, number of trials = 7, p-value = 0.08235
alternative hypothesis: true probability of success is less than 0.3
95 percent confidence interval:
0.0000000 0.3481637
sample estimates:
probability of success

With no woman hired that year, we can still pretend that there was some kind of “positive discrimination“. An note that we do accept – with more confidence – the assumption of “positive discrimination” if we look at all universities together,

> prop.test(5+2,11+4+4+3+5+7,.3,alternative="less")
1-sample proportions test with continuity correction
data:  5 + 2 out of 11 + 4 + 4 + 3 + 5 + 7, null probability 0.3
X-squared = 1.021, df = 1, p-value = 0.1561
alternative hypothesis: true p is less than 0.3
95 percent confidence interval:
0.0000000 0.3556254
sample estimates:
> binom.test(5+2,11+4+4+3+5+7,.3,alternative="less")
Exact binomial test
data:  5 + 2 and 11 + 4 + 4 + 3 + 5 + 7
number of successes = 7, number of trials = 34, p-value = 0.1558
alternative hypothesis: true probability of success is less than 0.3
95 percent confidence interval:
0.0000000 0.3521612
sample estimates:
probability of success

So obviously, with small sample, almost anything can be claimed !

Playing with quantiles, part 1

A standard idea in extreme value theory (see e.g. here, in French unfortunately) is that to estimate the 99.5% quantile (say), we just need to estimate a quantile of level 95% for observations exceeding the 90% quantile.

In extreme value theory, we assume that the 90% quantile (of the initial distribution) can be obtained easily, e.g. the empirical quantile, and then, for the exceeding observations, we fit a Pareto distribution (a Generalized Pareto one to be precise), and get a parametric quantile for the 95% quantile. I.e.


which can be written


So, an estimation of the cumulative distribution function is


and if we invert it, we get the popular expression for high level quantiles,


Hence, we do not really care about observations in the core of the distribution.

And I was wondering if this can be transposed with quantile regressions. Hence, I would like to get a quantile regression of level 90% (say) of http://freakonometrics.blog.free.fr/public/perso2/qqq06.gif given http://freakonometrics.blog.free.fr/public/perso2/qqqo5.gif, based on observations http://freakonometrics.blog.free.fr/public/perso2/qqq04.gif‘s, but all observations such that http://freakonometrics.blog.free.fr/public/perso2/qqq07.gif for some http://freakonometrics.blog.free.fr/public/perso2/qqq08.gif are missing. More precisely, I have the following sample (here half of the observations are missing),

Assume that we know that I have observations below the http://freakonometrics.blog.free.fr/public/perso2/qqq06.gif quantile of level 25%, and above the http://freakonometrics.blog.free.fr/public/perso2/qqq06.gif quantile of level 75%.
If I want to get the 90% quantile regression, and the 10% quantile, the code is simply,

Sigma <- matrix(c(1,r,r,1), 2, 2)
points(X[I],Y[I],col="light blue",cex=.7)

The graph is the following

Dotted lines – in black – are theoretical lines (if I had all observations), and plain lines are (where half of the sample if missing). Instead of a standard linear quantile regression, it is also possible to try a spline regression,

So obviously, if I miss something in the middle, that’s no big deal, doted and plain lines are here extremely close.
But what if observations http://freakonometrics.blog.free.fr/public/perso2/qqqo5.gif and http://freakonometrics.blog.free.fr/public/perso2/qqq06.gif were correlated ? Consider a Gaussian random vector http://freakonometrics.blog.free.fr/public/perso2/qqq09.gif with correlation http://freakonometrics.blog.free.fr/public/perso2/qqq10.gif (here 0.

It looks like we overestimate the slope for high quantile, but not for lower quantiles. So if observations are correlated, we have to be cautious with that technique.
But why could that be interesting ? Well, because I wanted to run a quantile regression on marathon results. But I could not get the overall dataset (since I had to import observations manually, and I have to admit that it was a bit boring). So I extracted finish times of the first 10% athletes, and the latest 10%. And I was wondering if it was enough to look at the 5% and 95% quantiles, based on the age of the runner… To be continued.

A Million Random Digits: review of reviews

Recently on his blog (here), Robin mentioned an amazing book, called “A Million Random Digits” published by RAND corporation. The book was initially published in 1955, but RAND published a nice (and expensive) second edition.

A great thing is that on Amazon, there are several extremely interesting reviews of the book. E.g.

4.0 out of 5 stars Didn’t like the ending, February 10, 2009  By Damien Katz

Even though I didn’t really see it coming, the ending was kind of anti-climatic. But overall the book held my attention and I really liked the “10034 56429 234088” part. It’s nice to know I’m not the only one who feels that way.

5.0 out of 5 stars I found a typo, September 14, 2007  By fanfan

To whom do I write to report typographical errors? I noticed that the first “7” on the third line page 48 should be a “3”. The “7” that’s printed there now isn’t random. Other than that, this is really an excellent book.

5.0 out of 5 stars Superb and original plot, April 21, 2007  By Herr Tarquin Biskuitfaß

This one has a very unpredictable plot, sublime character development in a style that stubbornly defies any sort of development in its rare and iconoclastic brilliance, and is told remarkably with numbers instead of letters. Take, for example, this passage on page 202, “98783 24838 39793 80954”. I’m speechless. The symmetry is reminiscent of the I Ching, and it approaches a rare spiritual niveau lacking in American literature. It not only reads well, but it looks great too. I have a tattoo of page 214 on my arm, and I’m hoping to get 202 on my belly to celebrate my next birthday. It is an injustice that Rand Corporation has not received the Nobel Prize for Literature, nor even a Pulitzer.

3.0 out of 5 stars A serious reference work?, October 16, 2006  By BJ

For a supposedly serious reference work the omission of an index is a major impediment. I hope this will be corrected in the next edition.

1.0 out of 5 stars Not Nearly A Million, September 3, 2006  By Liron

This book does not even come close to delivering on its promise of one million random digits. My expectations were high after reading the first sentence, which contained ten unique digits. However, the author seems to have exhasted his creativity in this initial burst, because the other 99.999% of the book is filler in which those same ten digits are shamelessly reused!  If you are looking for a larger offering of numerals in various bases, I highly recommend “Peter Rabbit’s ABC and 123”.

3.0 out of 5 stars Wait for the audiobook version, October 19, 2006  By R. Rosini “Newtype”

While the printed version is good, I would have expected the publisher to have an audiobook version as well. A perfect companion for one’s Ipod.

5.0 out of 5 stars Wait for it…, February 10, 2009  By Cranky Yankee

It started off slow, single digit slow in the beginning but I stuck with it. I eventually learned all about the different numbers, 1,2,3,4,5,6,7,8,9 and 0 and their different combinations.  The author introduced them all a bit too quickly for my taste. I would have been perfectly happy with just 1,2,3,4 and 5 for the first 20,000 digits, but then again, I’m not a famous random-number author, am I?  After a while, patterns emerged and the true nature of the multiverse was revealed to me, and the jokes were kinda funny. I don’t want to spoil anything but you will LOVE the twist ending!  Like 4352204 said to 64231234, “2242 6575 0013 2829!”

Ok, I have to admit I tried to check a few of them (that’s my freaky part). For instance the first one is a fake: the two first numbers – for instance – never show up together (consecutively),

> DIGIT=read.table("
+ http://freakonometrics.blog.free.fr/public/data/digits.txt")
> DIGIT=DIGIT[,2:11]
> k=1
> I=apply(DIGIT[,1:2]==c(10034,56429),1,sum)==2
> for(k in 2:9){
+ I=cbind(I,apply(DIGIT[,k+0:1]==c(10034,56429),1,sum)==2)
+ }
> I0=which(apply(I,1,sum)>0)
> DIGIT[I0,]
 [1] V2  V3  V4  V5  V6  V7  V8  V9  V10 V11
<0 rows> (or 0-length row.names)

Nevertheless, I did have some fun reading those reviews. About the book, unfortunately I have to confess I stopped after 99998 appeared (the first time).

when Nuns or Hells Angels get in a plane

Today, at lunch, Matthieu told us a nice story (or call it a paradox if you like) about the probability to find you seat empty when you get in a place. 

  • a plane full of nuns

Assume that you are in the line to get in the airplane, you are the 100th in the line. The first one is scatter brained, he has his head in the clouds, and when he get in the airplane, he cannot remember where he should seat. His strategy is then extremely simple: he seats randomly in the plane. So he picks up randomly a seat, and he waits.

Then come 98 nuns (one by one). And nuns are extremely polite: if there is someone in their seat (the one that is on the ticket they have) then they do not complain, and pick up another seat randomly (among those available, of course). Then you arrive. The question is simple: what is the probability that someone is seated at your seat ?

Any idea…?

Maybe I should give more time to do the maths… and tell another story…

  • a plane full of Hells Angels

Consider almost the same problem as the one mentioned above. Except that now, it is not 98 nuns that are getting in the plane, but 98 Hells Angels. So the problem here is that Hells Angels are slightly less polite than nuns. When they find someone seating on the seat they should have, they do not shyly move to another seat, but they grunt and then our scatter brained man (who is actually seating in their seat) has to move somewhere else. And the question is the same: you are the 100th person to get in the plane, what is the probability that someone is seated at your seat ?Any idea….?

The important point is that the problem is exactly the same (at least from a mathematical point of view, maybe not for the stewardess, or from the guy who enter first in the plane). The point is that, at each time, there could be only one person (or less) seating in a seat which is not his or hers (in the sense that if we compare the list of the passenger at any time, and the list of seats taken, there should be only one – or less – difference). The difference in the two story is that in the first case, it will be a nun, while in the second one, it will be our shy guy.

  • Let us run simulations

If we do not see how to get that probability analytically, let us run some R code,

> set.seed(1)
> n=100; TEST=rep(NA,100000)
> for(s in 1:100000){
+ OCCUPIED[sample(1:n,size=1)]=TRUE
+ for(j in 2:(n-1)){
+ if(OCCUPIED[j]==TRUE){OCCUPIED[sample(FREE,size=1)]=TRUE}
+ }
+ }
> mean(TEST)
[1] 0.49878

Here, we clearly see that the problem is the same (either with nuns or Hells Angels): we do not care about who will change his/her seat, but we just look at seats that are available… So the program is valid for the two problems (and the solution will then be the same). Another point is that the probability looks extremely simple: one over two !

  • an analytical expression

Consider the Hells Angels problem (for notations). Let http://freakonometrics.blog.free.fr/public/perso2/nonnes1.gif denote the probability that, at time http://freakonometrics.blog.free.fr/public/perso2/nonne6.gif, our shy guy is sitting in my seat. When he gets in the plane, the probability that he gets to my seat is

Then, the probability that, after ith passenger’s entrance, our guy is sitting in my own seat is (since the initial proof was not correct, I remove it, see below for a nice proof) One can get that

So, we can get the probability that, when I get in, our guy is sitting in my own seat as


Hence, there is one chance out of two that my seat will be free… (which is what we got with Monte Carlo simulations).

But a faster proof is to observe that, in the Hells Angels case, our guy will be kicked out until he reaches either his seat, or mine. Since those two events are equiprobable, there is one chance out of two that he seats in my seat (and since no Hells Angel will seat in mine, only this first guy can). So the probability that someone is in my seat when I get in is one half.

Nice isn’t it ? And thanks Matthieu for the problem  (with his friend Claude’s solution with the Hells Angels, and Olivier and Renaud for their comments) !

Does the Student based confidence interval have any interest in practice ?

Friday in the course of statistics, we started the section on confidence interval, and like always, I got a bit confused with the degrees of freedom of the Student (should it be http://freakonometrics.blog.free.fr/public/perso2/IC-std-6.gif or http://freakonometrics.blog.free.fr/public/perso2/IC-std-5.gif ?) and which empirical variance (should we consider the one where we divide by http://freakonometrics.blog.free.fr/public/perso2/IC-std-6.gif or the one with http://freakonometrics.blog.free.fr/public/perso2/IC-std-5.gif ?).
And each time I start to get confused, the student obviously see it, and start to ask tricky questions… So let us make it clear now. The correct formula is the following: let




is a confidence interval for the mean of a Gaussian i.i.d. sample.
But the important thing is neither the n-1 that appear as degrees of freedom nor the http://freakonometrics.blog.free.fr/public/perso2/IC-std-6.gif that appear in the estimation of the standard error. Like always in mathematical result, the most important part of that result is not mentioned here: observations have to be i.i.d. and to be normally distributed. And not “almost” normally distributed….
Consider the following case: we have http://freakonometrics.blog.free.fr/public/perso2/IC-std-6.gif=20 observations that are almost normally distributed. Hence, I consider a student t distribution

n=20; X=rt(n,df=3)

An Anderson Darling normality test accepts a normal distribution in 2 cases out of 3.

for(s in 1:10000){
[1] 0.6799

With a true normal distribution if would be 95% of the cases, so in some sense, I can pretend that I generate almost normal samples.
For those samples, we can look at bounds of the 90% confidence interval for the mean, with three different formulas,


i.e. the correct one, or the one where I considered http://freakonometrics.blog.free.fr/public/perso2/IC-std-6.gif degrees of freedom instead of http://freakonometrics.blog.free.fr/public/perso2/IC-std-5.gif,


and the one were we condired a Gaussian quantile instead of a Student t one,


(and one might think to look at the non-unbiased estimator of the variance, also).
for(s in 1:10000){

One the graph below are plotted the distributions of the values obtained as lower bound of the 90% confidence interval,

(the curves with http://freakonometrics.blog.free.fr/public/perso2/IC-std-6.gif and http://freakonometrics.blog.free.fr/public/perso2/IC-std-5.gif degrees of freedom in quantiles are the same, here).
The dotted vertical line is the true lower bound of the 90%-confidence interval, given the true distribution (which was not a Gaussian one).
If I get back to the standard procedure in any statistical textbook, since the sample is almost Gaussian, the lower bound of the confidence interval should be (since we have a Student t distribution)

[1] -0.605381

instead of

[1] -0.5759391

(obtained with a Gaussian distribution instead of a Student one). Actually, both of them are quite different from the correct one which was


As I mentioned in a previous post (here), an important issue is that if we do not know a parameter and substitute an estimator, there is usually a cost (which means usually that the confidence interval should be larger). And this is what we observe here. From a teacher’s point of view, it is an important issue that should be mentioned in statistical courses….

But another important point is also that confidence interval is valid only if the underlying distribution is Gaussian. And not almost Gaussian, but really a Gaussian one.  So since with http://freakonometrics.blog.free.fr/public/perso2/IC-std-6.gif=20 observations everything might look Gaussian, I was wondering what should be done in practice… Because in some sense, using a Student quantile based confidence interval on some almost Gaussian sample is as wrong as using a Gaussian quantile based confidence interval on some Gaussian sample…

What is the optimal strategy to marry the best one ?

Valentine’s day is a nice opportunity to post on hot and sexy topics… Well, it’s also an important day that I should not miss, probably as much as Saint Patrick’smy wife’s birthday. And as I mentioned last week (here), it is difficult to get the distribution of the age of marriage on the internet… So maybe we can build up a small model, to understand when do girls decide to get married… Consider a young girl who knows that he will not meet thousands of men willing to marry her (actually, one can consider the opposite point of view, with young man who can find only https://f-origin.hypotheses.org/wp-content/blogs.dir/253/files/2015/12/mariage01.png girls willing to marry him, the problem can be assumed as symmetric, especially if I do not want to get feminist leagues on my back).

Assume that https://f-origin.hypotheses.org/wp-content/blogs.dir/253/files/2015/12/mariage01.png men agree to marry her. Of course, among those https://f-origin.hypotheses.org/wp-content/blogs.dir/253/files/2015/12/mariage01.png men, our girl wants to marry the “best” one (assume that men can be ranked objectively). Of course, she cannot meet the “best” guy immediately, so men are met randomly, and after each “interview“, either she reject him (forever, we assume she cannot get back and admit she made a mistake), or agree to marry him. An important assumption is that rejected men cannot be recalled.

From a mathematical point of view, we need to find the optimal stopping time. Here, the problem is slightly different compared with that one (with optimal time to get a bonus) or this one (with the optimal time to sit in a bar and have a beer). Here, we do not give “grades” to guy. The only thing that is observed is their relative ranks. Our girl cannot know if she’s meting the best of all men (out of https://f-origin.hypotheses.org/wp-content/blogs.dir/253/files/2015/12/mariage01.png), but she knows if this one is better than the ones she already met. From a mathematical point of view, at time https://f-origin.hypotheses.org/wp-content/blogs.dir/253/files/2015/12/mariage02.png, she knows the relative rank of https://f-origin.hypotheses.org/wp-content/blogs.dir/253/files/2015/12/mariage02.png (compared with the first https://f-origin.hypotheses.org/wp-content/blogs.dir/253/files/2015/12/mariage04.png), not his absolute rank. We also assume that https://f-origin.hypotheses.org/wp-content/blogs.dir/253/files/2015/12/mariage01.png is known.

The optimal strategy is that she has to reject automatically the first https://f-origin.hypotheses.org/wp-content/blogs.dir/253/files/2015/12/mariage04.png (some kind of calibration period), and then, starting at time https://f-origin.hypotheses.org/wp-content/blogs.dir/253/files/2015/12/mariage02.png, she will marry the best over the ones she has already met.
So assume that our girl already met https://f-origin.hypotheses.org/wp-content/blogs.dir/253/files/2015/12/mariage04.png guys, and decided to reject all of them. So now she’s trying to see if the https://f-origin.hypotheses.org/wp-content/blogs.dir/253/files/2015/12/mariage02.png can be the optimal time to stop, and start looking seriously ….For an arbitrary cut-off https://f-origin.hypotheses.org/wp-content/blogs.dir/253/files/2015/12/mariage02.png, the probability that the best applicant will show up at some time https://f-origin.hypotheses.org/wp-content/blogs.dir/253/files/2015/12/wedd01.gif is https://f-origin.hypotheses.org/wp-content/blogs.dir/253/files/2015/12/wedding01.gif




The https://f-origin.hypotheses.org/wp-content/blogs.dir/253/files/2015/12/wedd02.gif term is because there is only one “best” guy, and the https://f-origin.hypotheses.org/wp-content/blogs.dir/253/files/2015/12/wedd03.gifis the probability that he shows up at time https://f-origin.hypotheses.org/wp-content/blogs.dir/253/files/2015/12/wedd01.gif (this can be visualized below)

Thus, we can write




Thus, since the minimum of https://f-origin.hypotheses.org/wp-content/blogs.dir/253/files/2015/12/mariage18.png is obtained when https://f-origin.hypotheses.org/wp-content/blogs.dir/253/files/2015/12/mariage19.png, which is the optimal time to stop (or here to start seeking), i.e. 36.7%.

Hence, the best strategy is to reject automatically the first https://f-origin.hypotheses.org/wp-content/blogs.dir/253/files/2015/12/mariage20.png=37% of the candidates (which is the maximum value of the function above), and then to select the first one (if possible) that is better than all previous candidates.

Consider the following Monte Carlo procedure: assume that she rejects – automatically – the first https://f-origin.hypotheses.org/wp-content/blogs.dir/253/files/2015/12/mariage02.png (we consider a loop with all possible values for https://f-origin.hypotheses.org/wp-content/blogs.dir/253/files/2015/12/mariage02.png) and then gets married with the first one who is the best one she’s seen during the calibration period (or overall, which is the same),

for(m in 2:(n-1)){
WHICH=rep(NA,ns); MARIAGE=rep(0,ns)
for(s in 1:ns){
for(k in (m+1):n){

If we run it over all possible https://f-origin.hypotheses.org/wp-content/blogs.dir/253/files/2015/12/mariage02.png we get


The “distribution” (in green) can be seen as the probability to marry the guy of level https://f-origin.hypotheses.org/wp-content/blogs.dir/253/files/2015/12/mariage06.png, given that the first https://f-origin.hypotheses.org/wp-content/blogs.dir/253/files/2015/12/mariage02.png were rejected. The sum is not one since there is a non null probability to marry no one. Actually, the probability to get married is the following

The more she waits, the smaller the probability of getting married. But on the other hand, the more she waits, the “better” the husband…. On the graph below is plotted the rank of the guy she marries, if she gets married (it was actually the vertical plain line in red on the animation)

So there is a trade-off. If not getting married gives a 0 satisfaction (lower than finally marrying anyone), and if marrying the guy with rank https://f-origin.hypotheses.org/wp-content/blogs.dir/253/files/2015/12/mariage06.png gives here satisfaction https://f-origin.hypotheses.org/wp-content/blogs.dir/253/files/2015/12/mariage06.png ,we have

(it was the vertical doted line in red on the animation). So it looks like it is optimal to test the first 35-38% men, and then to marry the best one she finds (if he is better than the best one she met during the “testing” procedure). So our previous analysis looks correct…

Now to go further, I have to admit that this model is known in academic literature as the secretary problem. In 1989, Thomas Ferguson wrote a nice paper inStatistical Science entitled who solved the secretary problem (here). Anthony Mucci published also an article in the Annals of Probability on possible extensions, in 1973 (here), or Thomas Lorenzen (there) in 1981. This problem is definitively an interesting one !

When will my papers appear as references (if they do…) ?

Following my post on citations in academic journals, I wanted to go one step further in the understanding of the dynamic of citations. So here, the dataset looks like that: for each article, we have the name of the journal, the year of publication (also the title of the article, but here we do not use it, as well as the authors), and more interesting, the number of citations in journals (any kind of academic journal) published in 1996, 1997, …, 2011. Of course, articles published in 1999 might have their first citation only starting in 1999.

7188             1999
7191             1999
7195             1999
7188 Sequential inspection 
7191 On equitable resource approach
7195 Method for strategic  
                                        Authors     ISSN       Journal.Title
7188                         Yao D.D., Zheng S. 0030364X Operations Research
7191                                    Luss H. 0030364X Operations Research
7195 Seshadri S., Khanna A., Harche F., Wyle R. 0030364X Operations Research
     Volume Issue X139 DEV1996 DEV1997 DEV1998 DEV1999 DEV2000 DEV2001 DEV2002
7188     47     3    0       0       0       0       0       1       0       2
7191     47     3    0       0       0       0       0       0       2       0
7195     47     3    0       0       0       0       0       0       0       0
     DEV2003 DEV2004 DEV2005 DEV2006 DEV2007 DEV2008 DEV2009 DEV2010 DEV2011
7188       0       0       0       1       0       0       0       0       0
7191       3       4       1       4       4       8       4       6       1
7195       0       1       2       2       1       0       1       0       0
     X130655 X0 X130794
7188       4  0       4
7191      37  0      37
7195       7  0       7

The first step is to aggregate data, not to look at each article, but to look at all paper published in 1999 (say). And then, we look at the number in citations the year of publication, the year after, two years after, etc. It will appear in a triangle since if we look at articles published in 2010, there is only on possible year for citations (2010, since I removed 2011).

for(v in VOL){

Then, a standard idea (at least in insurance business, for claims payment development) is to consider that data are Poisson distributed, and the number of citations should depend on the year of publication of the article (a row effect) and the development (how many years after are we looking at, i.e. a column effect). More formally, let http://freakonometrics.blog.free.fr/public/perso2/citationD01.gif denote the number of citations of articles published year http://freakonometrics.blog.free.fr/public/perso2/citD02.gifduring year http://freakonometrics.blog.free.fr/public/perso2/citD03.gif (or after http://freakonometrics.blog.free.fr/public/perso2/citD04.gif years). And we assume that http://freakonometrics.blog.free.fr/public/perso2/citD05.gif

DEV =rep(1:15,each=15)

Since those are incremental values, in order to look at the paper of distribution, we need to sum them on a line. Thus, we can plot


(because we used factors, the first component has been replaced by the constant in the regression) or a normalized version to compare among journals. For instance, we would like to get 100 citations over 15 years.


And this is what we get, for several academic journals,

The pattern is rather different. For instance, in Health Economics, citations is a quick process: more than 40% of citations obtained over 15 years, were obtained during the first 4 years. On the other hand, in the Journal of Finance, it is much smaller: less than 15% of the citations were obtained during the first 4 years (on average). So it means that comparing citation based index (namely g or h) is a difficult exercise, especially with you researchers in different areas. The same gor index for young researcher, publishing either in Stochastic Processes and their Applications or Annals of Statistics, means that after 3 years, it can be 50% higher.

Now it is possible to look more into details, with below JRSS-B (on applied statistics). Note that here, citations come extremely slowly… to it might not be a good “strategy” (assuming that a researcher’s target is simply to get – quickly – a high citation index) for a young researcher to publish in JRSS-B

On the other hand, Biometrika is much faster (both are on applied statistics, but we’ve seen here that they were not in the same cluster)

We can also observe that Annals of Probability
and Stochastic Processes and their Applications

have (almost) similar patterns (SPA might be a bit faster). Anyway, I have been surprised to see that in theoretical journals citations are extremely fast. Especially if we compare with the Journal of Finance for instance

where I though citations were extremely fast. But I might have a non-correct interpretation: it might simply mean that in the Journal of Finance it is common to cite old papers (published 10 or 15 years ago), maybe more common that in stochastic processes…
Anyway, all suggestions about the interpretation are welcomed !

Think academic journals look the same ? Well, some do…

We have seen yesterday that finding an optimal strategy to publish is not that simple. And actually, it can be even more difficult in the case the journal rejects the paper (not because it is not correct, but because “it does not fit” with the standards, the quality of the journal, the audience, the editor’s mood, or whatever). The author has basically two choices,

  • forget about the article and move to something else (e.g. start a blog where he/she will be the author and the editor)
  • pretend that the article is worth publishing and then try to find another journal with similar interests

But this last choice is not that easy, since sometimes the author think that this journal was indeed the one that should publish it (e.g. all the articles on the subject have been published in that journal).
So I was wondering if there were clusters of journals, i.e. journals that publish almost the same kind of articles (so that next time one of my paper is rejected by the editor, I just go to for some journal in the same cluster).
So what I did is extremely simple: I looked at articles titles and looked for correlations between words frequency (I could have done that in key words, but I am not a big fan of those key words). I looked at 35 journals (that are somehow related to my areas of interest) and looked at titles of all articles published over the last 20 years. Then I kept the top 1000 of words, and I removed standard short words (“a“, “the“, “is“, etc). Actually, my top words looks like

"models" "model" "data" "estimation" "analysis" "time" 
"processes" "risk" "random" "stochastic" "regression" 
"market" "approach" "optimal" "based" "information" 
"evidence" "linear" "games" "bayesian" "theory" "effects"
"distribution" "multivariate" "tests" "markets" "markov"
"equilibrium" "dynamic" "process" "distributions" 
"application" "stock" "likelihood"

Then, I ran a principal component analysis on my dataset (containing 960 variables – here words – and 35 observations – here journal names).

res.pca = PCA(MATRICE, scale.unit=TRUE, ncp=5, 
plot.PCA(res.pca, axes=c(1, 2), choix="ind")

The projection of the journals on the first two axis looks like that

Here, we can clearly observe some clusters : on the up-left Journal of Finance and Journal of Banking and Finance (say financial journals) on the top-right Biometrika, Biometrics, Computational Statistics and Data Analysis and Journal of Econometrics (JASA is not far away, i.e. applied statistics journal). And below, on the right, Stochastic Processes and their Applications, Annals of Applied Probability, Journal of Applied Probability, Annals of Probability, Proceedings of AMS and Topology and Applications (ie more theoretical journal).
Note that the projection is rather robust: if I consider my first 200 words, the graph is the same

In order to go further in the interpretation, we can also plot variables, i.e. words from titles,

where we cannot distinguish anything. So if I just look at my top 30, here they are,

On top left we see market(s), risk or information; on top right analysis, effects, models or tests; while below we see Markov or process(es). And we can observe interesting facts: in finance in statistics, we talk about dynamics while in theoretical (mathematical) journal it is about processes.
But the goal was to find cluster, i.e. classes of journals that publish papers with similar titles.

cah = hclust(DISTANCE) 

Here we have

If some classes a rather natural (Journal of Applied Proba. and Advances in Applied Proba.or Economic Theory, Journal of Economic Theory and Journal of Mathematical Economics) some strong correlation are not simple to understand, (e.g. Insurance: Mathematics and Economics and Management Science or Annals of Statistics and the Journal of Multivariate Analysis).
Again, it might be possible to spend hours on the graphs, but if I want – someday – to submit something to one of those journals, I guess I have to stop here, and move to something else…


Open data might be a false good opportunity…

I am always surprised to see many people on Twitter tweeting about #opendata, e.g. @data4all, @usdatagov, @datapublicatwit, @ProPublica or @open3 among so many others… Initially, I was also very enthousiastic, but I have to admit thatopen data are rarely raw data. Which is what I am usually looking for, as a statistician…
Consider the following example: I was wondering (Valentine’s day is approaching)when will a man born in 1975 (say) get married – if he ever gets married ?More technically, I was looking for a distribution of the age of first marriage (given the year of birth), including the proportion of men that will never get married, for that specific cohort.

The only data I found on the internet is the following, on statistics.gov.uk/

Note that we can also focus on women (e.g. here). Is it possible to use that opendata to get an estimation of the distribution of first marriage for some specific cohort ? (and to answer the question I asked). Here, we have two dimensions: on line http://freakonometrics.free.fr/blog/latex/marriage01.gif, the year (of the marriage), and on column http://freakonometrics.free.fr/blog/latex/marriage02.gif, the age of the man when he gets married. Assume that those were rawdata, i.e. that we have the number of marriages of men of age http://freakonometrics.free.fr/blog/latex/marriage02.gif during the year http://freakonometrics.free.fr/blog/latex/marriage01.gif.

We are interested at a longitudinal lecture of the table, i.e. consider some man born year http://freakonometrics.free.fr/blog/latex/marriage03.gif, we want to estimate (or predict) the age he will get married, if he gets married. With raw data, we can do it… The first step is to build up triangles (to have a cohort vs. age lecture of the data), and then to consider a model, e.g.


where http://freakonometrics.free.fr/blog/latex/marriage05.gif is a year effect, and http://freakonometrics.free.fr/blog/latex/marriage06.gif is a cohort effect.

for(i in 1:16){
      [,1] [,2] [,3] [,4] [,5] [,6] [,7] [,8]
 [1,]   12  104  222  247  198  132   51   34
 [2,]    8   89  228  257  202  102   75   49
 [3,]    4   80  209  247  168  129   92   50
 [4,]    4   73  196  236  181  140   88   45
 [5,]    3   78  242  206  161  114   68   47
 [6,]   11  150  223  199  157  105   73   39
 [7,]   12  117  194  183  136   96   61   36
 [8,]   11  118  202  175  122   92   62   40
 [9,]   15  147  218  162  127   98   72   48
[10,]   20  185  204  171  138  112   82   NA
[11,]   31  197  240  209  172  138   NA   NA
[12,]   34  196  233  202  169   NA   NA   NA
[13,]   35  166  210  199   NA   NA   NA   NA
[14,]   26  139  210   NA   NA   NA   NA   NA
[15,]   18  104   NA   NA   NA   NA   NA   NA
[16,]   10   NA   NA   NA   NA   NA   NA   NA
glm(formula = Y ~ as.factor(X1) + as.factor(X2), family = "poisson")
Deviance Residuals: 
    Min       1Q   Median       3Q      Max  
-5.4502  -1.1611  -0.0603   1.0471   4.6214  
                    Estimate Std. Error z value Pr(>|z|)    
(Intercept)        2.8300461  0.0712160  39.739  < 2e-16 ***
as.factor(X1)1923  0.0099503  0.0446105   0.223 0.823497    
as.factor(X1)1928 -0.0212236  0.0449605  -0.472 0.636891    
as.factor(X1)1933 -0.0377019  0.0451489  -0.835 0.403686    
as.factor(X1)1938 -0.0844692  0.0456962  -1.848 0.064531 .  
as.factor(X1)1943 -0.0439519  0.0452209  -0.972 0.331082    
as.factor(X1)1948 -0.1803236  0.0468786  -3.847 0.000120 ***
as.factor(X1)1953 -0.1960149  0.0470802  -4.163 3.14e-05 ***
as.factor(X1)1958 -0.1199103  0.0461237  -2.600 0.009329 ** 
as.factor(X1)1963 -0.0446620  0.0458508  -0.974 0.330020    
as.factor(X1)1968  0.1192561  0.0450437   2.648 0.008107 ** 
as.factor(X1)1973  0.0985671  0.0472460   2.086 0.036956 *  
as.factor(X1)1978  0.0356199  0.0520094   0.685 0.493423    
as.factor(X1)1983  0.0004365  0.0617191   0.007 0.994357    
as.factor(X1)1988 -0.2191428  0.0981189  -2.233 0.025520 *  
as.factor(X1)1993 -0.5274610  0.3241477  -1.627 0.103689    
as.factor(X2)27    2.0748202  0.0679193  30.548  < 2e-16 ***
as.factor(X2)32    2.5768802  0.0667480  38.606  < 2e-16 ***
as.factor(X2)37    2.5350787  0.0671736  37.739  < 2e-16 ***
as.factor(X2)42    2.2883203  0.0683441  33.482  < 2e-16 ***
as.factor(X2)47    1.9601540  0.0704276  27.832  < 2e-16 ***
as.factor(X2)52    1.5216903  0.0745623  20.408  < 2e-16 ***
as.factor(X2)57    1.0060665  0.0822708  12.229  < 2e-16 ***
Signif. codes:  0 ‘***’ 0.001 ‘**’ 0.01 ‘*’ 0.05 ‘.’ 0.1 ‘ ’ 1 
(Dispersion parameter for poisson family taken to be 1)
    Null deviance: 5299.30  on 99  degrees of freedom
Residual deviance:  375.53  on 77  degrees of freedom
  (28 observations deleted due to missingness)
AIC: 1052.1
Number of Fisher Scoring iterations: 5

Here, we have been able to derive http://freakonometrics.free.fr/blog/latex/marriage12.gif and http://freakonometrics.free.fr/blog/latex/marriage13.gif, where now http://freakonometrics.free.fr/blog/latex/marriage14.gifdenotes the cohort.
We can now predict the number of marriages per year, and per cohort


Here, given the cohort http://freakonometrics.free.fr/blog/latex/marriage03.gif, the shape of http://freakonometrics.free.fr/blog/latex/marriage16.gif is the following

[1]  10.00000 222.94525 209.32773 159.87855 115.06971  42.59102
[7]  18.70168 148.92360
The errors (Pearson error) look like that

(where the darker the blue, the smaller the residuals, and the darker the red, the higher the residuals). Obviously, we are missing something here, like a diagonal effect. But this is not the main problem here…

I guess that study here is not valid. The problem is that we deal with open data, and numbers of marriages are not given here: what is given is a he proportion of marriage of men of age http://freakonometrics.free.fr/blog/latex/marriage02.gif during the year http://freakonometrics.free.fr/blog/latex/marriage01.gif, with a yearly normalization. There is a constraint on lines, i.e. we observe


so that


This is mentioned in the title

It is still possible to consider a Poisson regression on the http://freakonometrics.free.fr/blog/latex/marriage10.gif, but unfortunately, I do not think any interpretation is valid (unless demography did not change last century). For instance, the following sum


looks like that

 [1] 919.948 838.762 846.301 816.552 943.559 930.280 857.871 896.113
 [9] 905.086 948.087 895.862 853.738 826.003 816.192 813.974 927.437

i.e. if we look at the graph

But I do not think we can interpret that sum as the probability (if we divide by 1,000) that a man in that cohort gets married…. And more basically, I cannot do anything with that dataset…

So open data might be interesting. The problem is that most of the time, the data are somehow normalized (or aggregated). And then, it becomes difficult to use them…

So I will have to work further to be able to write something (mathematically valid) on marriage strategy before Valentine’s day…. to be continued.

Will I ever be a bayesian statistician ? (part 1)

Last week, during the workshop on Statistical Methods for Meteorology and Climate Change (here), I discovered how powerful bayesian techniques could be, and that there were more and more bayesian statisticians. So, if I was to fully understand applied statisticians in conferences and workshops, I really have to understand basics of bayesian statistics. I have published some time ago some posts on bayesian statistics applied to actuarial problems (here or there), but so far, I always thought that bayesian was a synonym for magician. To be honest, I am a Muggle, and I have not been trained as a bayesian. But I can be an opportunist…

So I decided to publish some posts on bayesian techniques, in order to prove that it is actually not that difficult to implement.

As far as I understand it, in bayesian statistics, the parameter is considered as a random variable (which is also the case, in classical mathematical statistics). But here, here assume that this parameter does have a parametric distribution….
Consider a classical statistical problem: assume we have a sample http://freakonometrics.free.fr/blog/bayy1.png i.i.d. with distribution http://freakonometrics.free.fr/blog/bayy2.png. Here we note


since parameter http://freakonometrics.free.fr/blog/bayyyyy001.png is a random variable. The idea is to assume that http://freakonometrics.free.fr/blog/bayyyyy001.png has a (so called a priori) distribution, e.g.


So far it was simple. The idea is then to consider the posterior distribution of http://freakonometrics.free.fr/blog/bayyyyy001.png, given the observations http://freakonometrics.free.fr/blog/bayyyyyy02.png. Thus, we need to compute the distribution of http://freakonometrics.free.fr/blog/bayyyyyy03.png which is here extremely simple (due to properties of the Gaussian distribution), i.e.




And them, it becomes extremely natural to consider http://freakonometrics.free.fr/blog/bayy20.png as an estimator of given our sample data (and thus, we also have a confidence interval since we know the distribution of http://freakonometrics.free.fr/blog/bayyyyy001.png given the observations http://freakonometrics.free.fr/blog/bayyyyyy02.png).
In order to be sure that we understood, consider now a heads and tails problem, i.e. http://freakonometrics.free.fr/blog/bayy5.png. Note, first, that \theta has support http://freakonometrics.free.fr/blog/bayy6.png. So we need a distribution on that support. Why not a beta distribution ? E.g.






From Bayes formula,


and we get easily


which is the density of a Beta distribution, i.e.


The estimator proposed is then the expected value of that conditional distribution,


Note that


Further, it is possible to derive confidence intervals using quantiles of the posterior distribution.
On the graphs below, we consider the following heads/tails sample

A first idea is to consider a uniform prior distribution.


A second idea is to consider an asymmetric beta distribution. First, with an asymmetry on the left,


or on the right

Finally a third idea is simply to get back to the standard Gaussian approximation,


If we compare the four models, we obtain (the plain black line is the Gaussian approximated distribution for the empirical mean), and red lines are obtained from prior beta distributions


The code to generate those graphs is the following
a1=1; b1=1
a2=4; b2=2
a3=2; b3=4
for(s in 1:100){
for(i in 1:s){lines(u,D1[1+i,],col=COULEUR[i])}
for(i in 1:s){lines(u,D2[1+i,],col=COULEUR[i])}
for(i in 1:s){lines(u,D3[1+i,],col=COULEUR[i])}
for(i in 1:s){lines(u,D4[1+i,],col=COULEUR[i])}

Here, we can see that computations are simple if the prior distribution has a distribution which is the conjugate of the observations’ distribution (see here for the list of prior and posterior standard distributions).
So far, I have two questions that naturally show up

  • is it possible to start with a neutral prior distribution, non informative ?
  • what if we are no longer working with conjugate distributions ?

Well, I guess I have to work a bit more to answer those questions…. to be continued

Warming in Paris: minimas versus maximas ?

Recently, I received comments (here and on Twitter) about my previous graphs on the temperature in Paris. I mentioned in a comment (there) that studying extremas (and more generally quantiles or interquantile evolution) is not the same as studying the variance. Since I am not a big fan of the variance, let us talk a little bit about extrema behaviour.

In order to study the average temperature it is natural to look at the linear (assuming that it is linear, but I proved that it could reasonably be assumed as linear in the paper) regression, i.e. least square regression, which gives the expected value. But if we care about extremes, or almost extremes, it is natural to look at quantile regression.

For instance, below, the green line is the least square regression, the red one is 97.5% quantile, and the blue on the 2.5% quantile regression.

It looks like the slope is the same, i.e. extremas are increasing as fast as the average…


(here I plot randomly some points to avoid a too heavy figure, since I have too many observations, but I keep all the observations in the regression !).

Now, if we look at the slope for different quantile level (Fig 6 in the paper, here, but on minimum daily temperature, here I look at average daily temperature), the interpretation is different.

for(i in seq(.005,.995,by=.005)){

with the following graph below,

for(i in seq(.005,.995,by=.005)){

Here, for minimas (quantiles associated to low probabilities, on the left), the trend has a higher slope than the average, so in some sense, warming of minimas is stronger than average temperature, and on other hand, for maximas (high probabilities on the right), the slope is smaller – but positive – so summer are warmer, but not as much as winters.
Note also that the story is different for minimal temperature (mentioned in the paper) compared with that study, made here on average daily temperature (see comments)… This is not a major breakthrough in climate research, but this is all I got…

More climate extremes, or simply global warming ?

In the paper on the heat wave in Paris (mentioned here) I discussed changes in the distribution of temperature (and autocorrelation of the time series).

During the workshop on Statistical Methods for Meteorology and Climate Change today (here) I observed that it was still an important question: is climate change affecting only averages, or does it have an impact on extremes ? And since I’ve seen nice slides to illustrate that question, I decided to play again with my dataset to see what could be said about temperature in Paris.
Recall that data can be downloaded here (daily temperature of the XXth century).


On the plot below, the dots in red are the annual maximum temperatures, while the dots in blue are the annual minimum temperature. The plain line is the regression line (based on the annual max/min), and the dotted lines represent the average maximum/minimum daily temperature (to illustrate the global tendency),

It is also possible to look at annual boxplot, and to focus either on minimas, or on maximas.


Plain dots are average temperature below the 5% quantile for minima, or over the 95% quantile for maxima (again with the regression line),

We can observe an increasing trend on the minimas, but not on the maximas !
Finally, an alternative is to remember that we focus on annual maximas and minimas. Thus, Fisher and Tippett theory (mentioned here) can be used. Here, we fit a GEV distribution on a blog of 10 consecutive years. Recall that the GEV distribution is

for(s in 1:10){

The location parameter http://freakonometrics.blog.free.fr/public/perso/gev4.png is the following, with on the left the minimas and on the right the maximas,

while the scale parameter http://freakonometrics.blog.free.fr/public/perso/gev3.png is

and finally the shape parameter http://freakonometrics.blog.free.fr/public/perso/gev2.png is

On those graphs, it is very difficult to say anything regarding changes in temperature extremes… And I guess this is a reason why there is still active research on that area…