# On Cochran Theorem (and Orthogonal Projections)

Cochran Theorem – from The distribution of quadratic forms in a normal system, with applications to the analysis of covariance published in 1934 – is probably the most import one in a regression course. It is an application of a nice result on quadratic forms of Gaussian vectors. More precisely, we can prove that if $\boldsymbol{Y}\sim\mathcal{N}(\boldsymbol{0},\mathbb{I}_d)$ is a random vector with $d$ $\mathcal{N}(0,1)$ variable then (i) if $A$ is a (squared) idempotent matrix $\boldsymbol{Y}^\top A\boldsymbol{Y}\sim\chi^2_r$ where $r$ is the rank of matrix $A$, and (ii) conversely, if $\boldsymbol{Y}^\top A\boldsymbol{Y}\sim\chi^2_r$ then $A$ is an idempotent matrix of rank $r$. And just in case, $A$ is an idempotent matrix means that $A^2=A$, and a lot of results can be derived (for instance on the eigenvalues). The prof of that result (at least the (i) part) is nice: we diagonlize matrix $A$, so that $A=P\Delta P^\top$, with $P$ orthonormal. Since $A$ is an idempotent matrix observe that$$A^2=P\Delta P^\top=P\Delta P^\top=P\Delta^2 P^\top$$where $\Delta$ is some diagonal matrix such that $\Delta^2=\Delta$, so terms on the diagonal of $\Delta$ are either $0$ or $1$‘s. And because the rank of $A$ (and $\Delta$) is $r$ then there should be $r$ $1$‘s and $d-r$ $1$‘s. Now write$$\boldsymbol{Y}^\top A\boldsymbol{Y}=\boldsymbol{Y}^\top P\Delta P^\top\boldsymbol{Y}=\boldsymbol{Z}^\top \Delta\boldsymbol{Z}$$where $\boldsymbol{Z}=P^\top\boldsymbol{Y}$ that satisfies$\boldsymbol{Z}\sim\mathcal{N}(\boldsymbol{0},PP^\top)$ i.e. $\boldsymbol{Z}\sim\mathcal{N}(\boldsymbol{0},\mathbb{I}_d)$. Thus $$\boldsymbol{Z}^\top \Delta\boldsymbol{Z}=\sum_{i:\Delta_{i,i}-1}Z_i^2\sim\chi^2_r$$Nice, isn’t it. And there is more (that will be strongly connected actually to Cochran theorem). Let $A=A_1+\dots+A_k$, then the two following statements are equivalent (i) $A$ is idempotent and $\text{rank}(A)=\text{rank}(A_1)+\dots+\text{rank}(A_k)$ (ii) $A_i$‘s are idempotents, $A_iA_j=0$ for all $i\neq j$.

Now, let us talk about projections. Let $\boldsymbol{y}$ be a vector in $\mathbb{R}^n$. Its projection on the space $\mathcal V(\boldsymbol{v}_1,\dots,\boldsymbol{v}_p)$ (generated by those $p$ vectors) is the vector $\hat{\boldsymbol{y}}=\boldsymbol{V} \hat{\boldsymbol{a}}$ that minimizes $\|\boldsymbol{y} -\boldsymbol{V} \boldsymbol{a}\|$ (in $\boldsymbol{a}$). The solution is$$\hat{\boldsymbol{a}}=( \boldsymbol{V}^\top \boldsymbol{V})^{-1} \boldsymbol{V}^\top \boldsymbol{y} \text{ and } \hat{\boldsymbol{y}} = \boldsymbol{V} \hat{\boldsymbol{a}}$$
Matrix $P=\boldsymbol{V} ( \boldsymbol{V}^\top \boldsymbol{V})^{-1} \boldsymbol{V}^\top$ is the orthogonal projection on $\{\boldsymbol{v}_1,\dots,\boldsymbol{v}_p\}$ and $\hat{\boldsymbol{y}} = P\boldsymbol{y}$.

Now we can recall Cochran theorem. Let $\boldsymbol{Y}\sim\mathcal{N}(\boldsymbol{\mu},\sigma^2\mathbb{I}_d)$ for some $\sigma>0$ and $\boldsymbol{\mu}$. Consider sub-vector orthogonal spaces $F_1,\dots,F_m$, with dimension $d_i$. Let $P_{F_i}$ be the orthogonal projection matrix on $F_i$, then (i) vectors $P_{F_1}\boldsymbol{X},\dots,P_{F_m}\boldsymbol{X}$ are independent, with respective distribution $\mathcal{N}(P_{F_i}\boldsymbol{\mu},\sigma^2\mathbb{I}_{d_i})$ and (ii) random variables $\|P_{F_i}(\boldsymbol{X}-\boldsymbol{\mu})\|^2/\sigma^2$ are independent and $\chi^2_{d_i}$ distributed.

We can try to visualize those results. For instance, the orthogonal projection of a random vector has a Gaussian distribution. Consider a two-dimensional Gaussian vector

library(mnormt) r = .7 s1 = 1 s2 = 1 Sig = matrix(c(s1^2,r*s1*s2,r*s1*s2,s2^2),2,2) Sig Y = rmnorm(n = 1000,mean=c(0,0),varcov = Sig) plot(Y,cex=.6) vu = seq(-4,4,length=101) vz = outer(vu,vu,function (x,y) dmnorm(cbind(x,y), mean=c(0,0), varcov = Sig)) contour(vu,vu,vz,add=TRUE,col='blue') abline(a=0,b=2,col="red")

Consider now the projection of points $\boldsymbol{y}=(y_1,y_2)$ on the straight linear with directional vector $\overrightarrow{\boldsymbol{u}}$ with slope $a$ (say $a=2$). To get the projected point $\boldsymbol{x}=(x_1,x_2)$ recall that $x_2=ay_1$ and $\overrightarrow{\boldsymbol{x},\boldsymbol{y}}\perp\overrightarrow{\boldsymbol{u}}$. Hence, the following code will give us the orthogonal projections

p = function(a){ x0=(Y[,1]+a*Y[,2])/(1+a^2) y0=a*x0 cbind(x0,y0) }

with

P = p(2) for(i in 1:20) segments(Y[i,1],Y[i,2],P[i,1],P[i,2],lwd=4,col="red") points(P[,1],P[,2],col="red",cex=.7)

Now, if we look at the distribution of points on that line, we get… a Gaussian distribution, as expected,

z = sqrt(P[,1]^2+P[,2]^2)*c(-1,+1)[(P[,1]>0)*1+1] vu = seq(-6,6,length=601) vv = dnorm(vu,mean(z),sd(z)) hist(z,probability = TRUE,breaks = seq(-4,4,by=.25)) lines(vu,vv,col="red")

Or course, we can use the matrix representation to get the projection on $\overrightarrow{\boldsymbol{u}}$, or a normalized version of that vector actually

a=2 U = c(1,a)/sqrt(a^2+1) U [1] 0.4472136 0.8944272 matP = U %*% solve(t(U) %*% U) %*% t(U) matP %*% Y[1,] [,1] [1,] -0.1120555 [2,] -0.2241110 P[1,] x0 y0 -0.1120555 -0.2241110 

(which is consistent with our manual computation). Now, in Cochran theorem, we start with independent random variables,

Y = rmnorm(n = 1000,mean=c(0,0),varcov = diag(c(1,1)))

Then we consider the projection on $\overrightarrow{\boldsymbol{u}}$ and $\overrightarrow{\boldsymbol{v}}=\overrightarrow{\boldsymbol{u}}^\perp$

U = c(1,a)/sqrt(a^2+1) matP1 = U %*% solve(t(U) %*% U) %*% t(U) P1 = Y %*% matP1 z1 = sqrt(P1[,1]^2+P1[,2]^2)*c(-1,+1)[(P1[,1]>0)*1+1] V = c(a,-1)/sqrt(a^2+1) matP2 = V %*% solve(t(V) %*% V) %*% t(V) P2 = Y %*% matP2 z2 = sqrt(P2[,1]^2+P2[,2]^2)*c(-1,+1)[(P2[,1]>0)*1+1]

We can plot those two projections

plot(z1,z2)

and observe that the two are indeed, independent Gaussian variables. And (of course) there squared norms are $\chi^2_{1}$ distributed.

# On the conjugate function

In the MAT7381 course (graduate course on regression models), we will talk about optimization, and a classical tool is the so-called conjugate. Given a function $f:\mathbb{R}^p\to\mathbb{R}$ its conjugate is function $f^{\star}:\mathbb{R}^p\to\mathbb{R}$ such that $$f^{\star}(\boldsymbol{y})=\max_{\boldsymbol{x}}\lbrace\boldsymbol{x}^\top\boldsymbol{y}-f(\boldsymbol{x})\rbrace$$so, long story short, $f^{\star}(\boldsymbol{y})$ is the maximum gap between the linear function $\boldsymbol{x}^\top\boldsymbol{y}$ and $f(\boldsymbol{x})$.

Just to visualize, consider a simple parabolic function (in dimension 1) $f(x)=x^2/2$, then $f^{\star}(\color{blue}{2})$ is the maximum gap between the line $x\mapsto\color{blue}{2}x$ and function $f(x)$.

x = seq(-100,100,length=6001) f = function(x) x^2/2 vf = Vectorize(f)(x) fstar = function(y) max(y*x-vf) vfstar = Vectorize(fstar)(x)

We can see it on the figure below.

viz = function(x0=1,YL=NA){ idx=which(abs(x)<=3) par(mfrow=c(1,2)) plot(x[idx],vf[idx],type="l",xlab="",ylab="",col="blue",lwd=2) abline(h=0,col="grey") abline(v=0,col="grey") idx2=which(x0*x>=vf) polygon(c(x[idx2],rev(x[idx2])),c(vf[idx2],rev(x0*x[idx2])),col=rgb(0,1,0,.3),border=NA) abline(a=0,b=x0,col="red") i=which.max(x0*x-vf) segments(x[i],x0*x[i],x[i],f(x[i]),lwd=3,col="red") if(is.na(YL)) YL=range(vfstar[idx]) plot(x[idx],vfstar[idx],type="l",xlab="",ylab="",col="red",lwd=1,ylim=YL) abline(h=0,col="grey") abline(v=0,col="grey") segments(x0,0,x0,fstar(x0),lwd=3,col="red") points(x0,fstar(x0),pch=19,col="red") } viz(1)

or

viz(1.5)

In that case, we can actually compute $f^{\star}$, since $$f^{\star}(y)=\max_{x}\lbrace xy-f(x)\rbrace=\max_{x}\lbrace xy-x^2/2\rbrace$$The first order condition is here $x^{\star}=y$ and thus$$f^{\star}(y)=\max_{x}\lbrace xy-x^2/2\rbrace=\lbrace x^{\star}y-(x^{\star})^2/2\rbrace=\lbrace y^2-y^2/2\rbrace=y^2/2$$And actually, that can be related to two results. The first one is to observe that $f(\boldsymbol{x})=\|\boldsymbol{x}\|_2^2/2$ and in that case $f^{\star}(\boldsymbol{y})=\|\boldsymbol{y}\|_2^2/2$ from the following general result : if $f(\boldsymbol{x})=\|\boldsymbol{x}\|_p^p/p$ with $p>1$, where $\|\cdot\|_p$ denotes the standard $\ell_p$ norm, then $f^{\star}(\boldsymbol{y})=\|\boldsymbol{y}\|_q^q/q$ where$$\frac{1}{p}+\frac{1}{q}=1$$The second one is the conjugate of a quadratic function. More specifically if $f(\boldsymbol{x})=\boldsymbol{x}^{\top}\boldsymbol{Q}\boldsymbol{x}/2$ for some definite positive matrix $\boldsymbol{Q}$$f^{\star}(\boldsymbol{y})=\boldsymbol{y}^{\top}\boldsymbol{Q}^{-1}\boldsymbol{y}/2$. In our case, it was a univariate problem with $\boldsymbol{Q}=1$.

For the conjugate of the $\ell_p$ norm, we can use the following code to visualize it

p = 3 f = function(x) abs(x)^p/p vf = Vectorize(f)(x) fstar = function(y) max(y*x-vf) vfstar = Vectorize(fstar)(x) viz(1.5)

or

p = 1.1 f = function(x) abs(x)^p/p vf = Vectorize(f)(x) fstar = function(y) max(y*x-vf) vfstar = Vectorize(fstar)(x) viz(1, YL=c(0,10))

Actually, in that case, we almost visualize that if $f(x)=|x|$ then$$\displaystyle{f^{\star}\left(y\right)={\begin{cases}0,&\left|y\right|\leq 1\\\infty ,&\left|y\right|>1.\end{cases}}}$$

To conclude, another popular case, $f(x)=\exp(x)$ then$${\displaystyle f^{\star}\left(y\right)={\begin{cases}y\log(y)-y,&y>0\\0,&y=0\\\infty ,&y<0.\end{cases}}}$$We can visualize that case below

f = function(x) exp(x) vf = Vectorize(f)(x) fstar = function(y) max(y*x-vf) vfstar = Vectorize(fstar)(x) viz(1,YL=c(-3,3))

# De l’abus de notation dans les modèles de régression

De manière un peu rituelle, je commence toujours mon cours de régression en revenant sur un point important de la statistique : les abus de notation !  Car tout le monde utilise les mêmes lettres (surtout les grecques) pour désigner des objets de nature différente. Dans la majorité des livres, on pourra nous dire sur la même page que $\widehat{\theta}=2.35$ et que $\text{Var}(\widehat{\theta})=1.07$, autrement dit $\widehat{\theta}$ peut désigner en même temps un nombre (dans le premier cas) et une variable aléatoire (dans le second). C’est pour le moins déroutant ! En fait, la raison est assez simple. La statistique commence toujours par un échantillon $\{y_1,y_2,\cdots,y_n\}$, des données, des chiffres. Si on reste là, on fait du descriptif. L’étape classique est ensuite de supposer que les observations $y_i$ sont des réalisations de variables aléatoires $Y_i$, qu’on supposera bien souvent indépendantes et identiquement distribuées. Et $\widehat{\theta}$ sera alors une statistique, c’est à dire une fonction de mes observations. Je peux alors définir $\widehat{\theta}=t(y_1,\cdots,y_n)$ comme étant la statistique observée sur mon échantillon, mais je peux aussi considérer $\widehat{\theta}=t(Y_1,\cdots,Y_n)$, qui est alors une variable aléatoire, mais avec la même notation. Si on voulait aider à comprendre, on utiliserait $\widehat{\Theta}$, mais bon, les choses sont ce qu’elles sont… Et en économétrie, ça devient rapidement un cauchemar quand on commence à parler des résidus… Autre particularité en statistique, c’est que si on distingue l’espérance et la moyenne (empirique), on a un seul mot pour parler de la variance, que ce soit pour une variable aléatoire, ou un vecteur de $\mathbb{R}^n$. On aura ainsi $$\mathbb{E}[Y]=\int y dF(y)$$et$$\overline{y}=\widehat{\mathbb{E}}[\boldsymbol{y}]=\frac{1}{n}\sum_{i=1} y_i$$alors que$$\text{Var}[Y]=\int [y-\mathbb{E}[Y]]^2 dF(y)$$et$$\widehat{\text{Var}}[\boldsymbol{y}]=\frac{1}{n}\sum_{i=1} (y_i-\overline{y})^2$$

Considérons un problème de régression maintenant, avec un modèle de la forme $y_i=\boldsymbol{x}_i^\top\boldsymbol{\beta}+\varepsilon_i$. Ici, $\varepsilon_i$ est un nombre réel, inconnu. Dans une écriture matricielle, on a $\boldsymbol{y}=\boldsymbol{X}\boldsymbol{\beta}+\boldsymbol{\varepsilon}$, où cette fois $\boldsymbol{\varepsilon}$ est un vecteur de $\mathbb{R}^n$ (et oui, je suis désolé mais ici $\boldsymbol{X}$ désigne la matrice des covariates, et non pas un vecteur aléatoire… je ferais un billet un jour pour parler du fait que parfois on dit que les $\boldsymbol{x}$ sont donnés et des fois – comme on conditionne suivant $\boldsymbol{X}$, autrement dit, on les voit comme aléatoires). On peut parfois faire une hypothèse quant à la distribution des résidus. Autrement dit, les $\varepsilon_i$ sont vues comme des réalisations de variables aléatoires $\varepsilon_i$, ainsi que $\boldsymbol{\varepsilon}$. On notera ainsi $\boldsymbol{\varepsilon}\sim\mathcal{N}(\boldsymbol{0},\boldsymbol{\Sigma})$. Ah oui, autre point juste pour perdre les élèves : $\text{Var}(\boldsymbol{\varepsilon})=\boldsymbol{\Sigma}$ alors que $\text{Var}(\varepsilon_i)=\sigma^2$… Bon, ici comme on suppose les observations indépendentes, et identiquement distribuées, on supposera que $\text{Var}(\boldsymbol{\varepsilon})=\boldsymbol{\Sigma}=\sigma^2\mathbb{I}$.

Encore une fois, $\boldsymbol{\varepsilon}$ est (par définition) non observable. Par contre, on peut estimer ces résidus : à partir d’un estimateur $\widehat{\boldsymbol{\beta}}$ de $\boldsymbol{\beta}$, on peut définir $$\widehat{\boldsymbol{\varepsilon}}=\boldsymbol{y}-\widehat{\boldsymbol{y}}=\boldsymbol{y}-\boldsymbol{x}^\top\widehat{\boldsymbol{\beta}}$$Histoire de clarifier, je vais plutôt noter  $\widehat{\boldsymbol{e}}$ ces résidus estimés, en utilisant l’estimateur par moindres carrés de $\boldsymbol{\beta}$. On peut noter que $\widehat{\boldsymbol{e}}=(\mathbb{I}-\boldsymbol{H})\boldsymbol{y}$ où classiquement $\boldsymbol{H}=\boldsymbol{X}(\boldsymbol{X}^\top\boldsymbol{X})^{-1}\boldsymbol{X}^\top$ est la matrice de projection sur l’espace engendré par toutes les combinaisons linéaires des variables explicatives. Mais là encore, on peut voir le vecteur (numérique) $\widehat{\boldsymbol{e}}$ comme la réalisation d’une variable aléatoire $\widehat{\boldsymbol{E}}$. En particulier, $$\widehat{\boldsymbol{E}}=(\mathbb{I}-\boldsymbol{H})\boldsymbol{Y}=(\mathbb{I}-\boldsymbol{H})\boldsymbol{\varepsilon}$$$\boldsymbol{\varepsilon}$ est notre vecteur aléatoire, centré, de matrice de variance-covariance $\text{Var}(\boldsymbol{\varepsilon})=\sigma^2\mathbb{I}$. On peut alors en déduire que$$\mathbb{E}[\widehat{\boldsymbol{E}}]=(\mathbb{I}-\boldsymbol{H})\mathbb{E}[\boldsymbol{\varepsilon}]=\boldsymbol{0}$$et$$\text{Var}[\widehat{\boldsymbol{E}}]=(\mathbb{I}-\boldsymbol{H})\text{Var}[\boldsymbol{\varepsilon}](\mathbb{I}-\boldsymbol{H})^\top=\sigma^2(\mathbb{I}-\boldsymbol{H})$$(car $(\mathbb{I}-\boldsymbol{H})$ est idenpotent). Cette dernière relation est particulièrement importante, car on notera que $\text{Var}(\widehat{\boldsymbol{E}})\neq\sigma^2\mathbb{I}$. En particulier, si on prend un résidu estimé au hasard $\text{Var}(\widehat{E}_i)=\sigma^2(1-\boldsymbol{H}_{i,i})$ (on avait parlé longuement de $\boldsymbol{H}_{i,i}$ dans un billet récent, on leverage, en particulier on avait vu que $\boldsymbol{H}_{i,i}\in[0,1]$ (on avait discuté la borne inférieur, qui peut être améliorée, en fait $\boldsymbol{H}_{i,i}\in(0,1]$) de telle sorte que $\text{Var}(\widehat{E}_i)\leq\sigma^2$. Si on poursuit un peu, on peut regarder la somme des carrés estimés, et noter que$$\mathbb{E}\big[\sum_{i=1}^n \widehat{E}_i^2\big]=\mathbb{E}[\text{trace}( \widehat{\boldsymbol{E}}\widehat{\boldsymbol{E}}^\top)] =\text{trace}(\mathbb{E}[\text{trace}( \widehat{\boldsymbol{E}}\widehat{\boldsymbol{E}}^\top])$$i.e.$$\mathbb{E}\big[\sum_{i=1}^n \widehat{E}_i^2\big]=\sigma^2\text{trace}(\mathbb{I}-\boldsymbol{H})$$or $\text{trace}(\mathbb{I}-\boldsymbol{H})=n-p$ donc$$\widehat{\sigma}^2=\frac{1}{n-p}\sum_{i=1}^n \widehat{E}_i^2$$est un estimateur sans biais de $\sigma^2$. Et classiquement, on considèrera les résidus Studentisés$$\widehat{R}_i=\frac{\widehat{E}_i}{\widehat{\sigma}\sqrt{1-\boldsymbol{H}_{i,i}}}$$Si je voulais résumer un peu, on pourrait dire que$$\text{Var}(\boldsymbol{E})=\sigma^2\mathbb{I}$$$$\widehat{\text{Var}}(\boldsymbol{E})=\widehat{\sigma}^2\mathbb{I}$$$$\text{Var}(\widehat{\boldsymbol{E}})=\sigma^2(\mathbb{I}-\boldsymbol{H})$$$$\widehat{\text{Var}}(\widehat{\boldsymbol{E}})=\widehat{\sigma}^2(\mathbb{I}-\boldsymbol{H})$$En espérant que ça clarifie un peu…(?)

# On leverage

Last week, in our STT5100 (applied linear models) class, I’ve introduce the hat matrix, and the notion of leverage. In a classical regression model, $\boldsymbol{y}=\boldsymbol{X}\boldsymbol{\beta}$ (in a matrix form), the ordinary least square estimator of parameter $\boldsymbol{\beta}$ is $$\widehat{\boldsymbol{\beta}}=(\boldsymbol{X}^\top\boldsymbol{X})^{-1}\boldsymbol{X}^\top\boldsymbol{y}$$The prediction can then be written$$\widehat{\boldsymbol{y}}=\boldsymbol{X}\widehat{\boldsymbol{\beta}}=\underbrace{\color{blue}{\boldsymbol{X}(\boldsymbol{X}^\top\boldsymbol{X})^{-1}\boldsymbol{X}^\top}}_{\color{blue}{\boldsymbol{H}}}\boldsymbol{y}$$where $\color{blue}{\boldsymbol{H}}$ is called the hat matrix.

The matrix is idempotent, i.e. $$\boldsymbol{H}^2={\boldsymbol{X}(\boldsymbol{X}^\top\boldsymbol{X})^{-1}\textcolor{grey}{\boldsymbol{X}^\top{\boldsymbol{X}}(\boldsymbol{X}^\top\boldsymbol{X})^{-1}}\boldsymbol{X}^\top}={\boldsymbol{X}(\boldsymbol{X}^\top\boldsymbol{X})^{-1}\boldsymbol{X}^\top}=\boldsymbol{H}$$so it can be interpreted as a projection matrix. Furthermore, since$\boldsymbol{H}\boldsymbol{X}=\boldsymbol{X}$ (just do the maths), the projection is on a subspace that contains all the linear combinations of columns of $\boldsymbol{X}$. One can also observe that $\mathbb{I}-\boldsymbol{H}$ is also a projection matrix. And we can write$$\boldsymbol{y}=\underbrace{\boldsymbol{H}\boldsymbol{y}}_{\widehat{\boldsymbol{y}}}+\underbrace{(\mathbb{I}-\boldsymbol{H})\boldsymbol{y}}_{\widehat{\boldsymbol{\varepsilon}}}$$where $\widehat{\boldsymbol{y}}$ is the orthogonal projection of $\boldsymbol{y}$ on the (linear) space of linear combinations of columns of $\boldsymbol{X}$, and $\widehat{\boldsymbol{y}}\perp\widehat{\boldsymbol{\varepsilon}}$, which gives the classical interpretation of residuals, being unpredictible (at least with a linear model using variables $\boldsymbol{X}$).

Let’s move a bit faster now (we’ve seen many other properties last week), and consider elements on the diagonal of matrix $\boldsymbol{H}$. Recall that we have

so entry $\boldsymbol{H}_{i,i}$ is a measure of the influence of entry $\boldsymbol{y}_i$ on its prediction latex]\widehat{\boldsymbol{y}}_i[/latex].

We have seen that$$\sum_{i=1}^n\boldsymbol{H}_{i,i}=\text{trace}(\boldsymbol{H})=\text{trace}(\boldsymbol{X}(\boldsymbol{X}^\top\boldsymbol{X})^{-1}\boldsymbol{X}^\top)$$which can be written$$\sum_{i=1}^n\boldsymbol{H}_{i,i}=\text{trace}\boldsymbol{X}^\top(\boldsymbol{X}(\boldsymbol{X}^\top\boldsymbol{X})^{-1})=\text{trace}(\mathbb{I})=p$$where classically $p=k+1$, where $k$ is the number of explanatory variables. Further, since $\boldsymbol{H}$ is idempotent, we can write (from $\boldsymbol{H}=\boldsymbol{H}^2$) that$$\boldsymbol{H}_{i,i}=\boldsymbol{H}_{i,i}^2 + \sum_{j\neq i}\boldsymbol{H}_{i,j}\boldsymbol{H}_{j,i}=\boldsymbol{H}_{i,i}^2 + \sum_{j\neq i}\boldsymbol{H}_{i,j}^2$$One the one hand, since the second term is positive $\boldsymbol{H}_{i,i}\geq\boldsymbol{H}_{i,i}^2$, i.e. $1\geq\boldsymbol{H}_{i,i}$. And since both terms are positive, then $\boldsymbol{H}_{i,i}\in[0,1]$. And there was a question in the course on the sharpeness of the bounds.

Using Anscombe’s dataset, we’ve seen that it was possible to get a leverage of 1. Using something rather similar

df = data.frame(x = c(rep(1,10),6), y = c(1:10,8)) plot(df)

we obtain

model = lm(y~x,data=df) abline(model,col="red",lwd=2) H = lm.influence(model)$hat plot(1:11,H,type="h") The very last observation, the one one the right, is here extremely influencial : if we remove it, the model is completely different ! And here, we reach the upper bound, $\boldsymbol{H}_{11,11}=1$. Observe that all other points are equally influencial, and because on the constraint on the trace of the matrix, $\boldsymbol{H}_{i,i}=1/10$ when $i\in\{1,2,\cdots,10\}$. Now, what about the lower bound ? In order to have some sort of “non-influencial” observations, consider the two following case. • the case where one observation (below the first one) is such that $\widehat{\boldsymbol{y}}_{i}=\boldsymbol{y}_{i}$ (perfect prediction) • the case where one observation (below the tenth one) is such that $\boldsymbol{x}_{i}=\overline{\boldsymbol{x}}$ and $\boldsymbol{y}_{i}=\overline{\boldsymbol{y}}$ (from the first order condition – or normal equation), the fitted regression line always go through point $(\overline{\boldsymbol{x}},\overline{\boldsymbol{y}})$ Let us move two observations from our dataset, mean(c(4,rep(1,8),6)) [1] 1.8 df = data.frame(x = c(4,rep(1,8),6,1.8), y = c(predict(model,newdata=data.frame(x=4)), 2:9,8, predict(model,newdata=data.frame(x=1.8)))) We now have If we compute the leverages, we obtain model = lm(y~x,data=df) H = lm.influence(model)$hat plot(1:11,H,type="h")

so, for the first observation, its leverage actually increased (the blue part), and for the tenth one, we have the lowest influence, but it is not zero. Is it possible to reach zero ?

Here, observe that for the tenth observation, $\boldsymbol{H}_{i,i}=1/n$. And actually, that’s the best we can do… We can prove that, in the case of a simple regression (as above)$$\boldsymbol{H}_{i,i}=\frac{1}{n}+\frac{(x_i-\overline{x})^2}{n\text{Var}(x)}$$which is minimum when $x_i=\overline{x}$, and then $\boldsymbol{H}_{i,i}=1/n$, otherwise $\boldsymbol{H}_{i,i}>1/n$. And this property is also valid in a multiple regression (as soon as an intercept is included in the regression – which should always be the case). To prove that result, let $\tilde{\boldsymbol{X}}$ denote the matrix of centered variables $\boldsymbol{X}$, then we can prove that $$\boldsymbol{H}_{i,i}=\frac{1}{n}+\big[\tilde{\boldsymbol{X}}(\tilde{\boldsymbol{X}}^\top\tilde{\boldsymbol{X}})^{-1}\tilde{\boldsymbol{X}}^\top\big]_{i,i}$$(which is basically a matrix version of the previous equation).

I can maybe add another comment on Anscombe’s data. We’ve seen that on the right that we did reach 1. But I did not prove it. One way to prove it is actually to focus on the remaining $n-1$ points, on the left. Those have all the same $x$ values. We can prove that if $\boldsymbol{X}_{i_1}=\boldsymbol{X}_{i_2}$, then $$\boldsymbol{H}_{i_1,i_2}=\boldsymbol{X}_{i_1}^\top(\boldsymbol{X}^\top\boldsymbol{X})^{-1}\boldsymbol{X}_{i_2}=\boldsymbol{H}_{i_1,i_1}$$hence, using the relationship obtained since the hat matrix is idempotent$$\boldsymbol{H}_{i_1,i_1}=2\boldsymbol{H}_{i_1,i_1}^2+\sum_{j\notin\{i_1,i_2\}}\boldsymbol{H}_{i_1,j}^2$$thus, we now have$$\boldsymbol{H}_{i_1,i_1}\big(1-2\boldsymbol{H}_{i_1,i_1}\big)>0$$i.e. $\boldsymbol{H}_{i_1,i_1}\in[0,1/2]$, where the upper bound becomes $1/(n-1)$ “duplicates”. So for $n-1$ $\boldsymbol{H}_{i,i}$‘s, we have values below $1/(n-1)$, the last one should be below $1$ and the sum has to be $k=2$ . So we have the value of the $n$ $\boldsymbol{H}_{i,i}$‘s.

# Insurance data science : Networks

At the Summer School of the Swiss Association of Actuaries, in Lausanne, I will start talking about networks and insurance this Friday. Slides are available online

# Insurance data science : Text

At the Summer School of the Swiss Association of Actuaries, in Lausanne, I will start talking about text based data and NLP this Thursday. Slides are available online

Ewen Gallic (AMSE) will present a tutorial on tweets. I can upload a few additional slides on LSTM (recurrent neural nets)

# Insurance data science : Pictures

At the Summer School of the Swiss Association of Actuaries, in Lausanne, following the part of Jean-Philippe Boucher (UQAM) on telematic data, I will start talking about pictures this Wednesday. Slides are available online

Ewen Gallic (AMSE) will present a tutorial on satellite pictures, and a simple classification problem, related to Alzeimher detection.

We will try to identify what is on the following pictures, starting with the car

(we will see that the car is indeed identified)

a skier,

and a fire,

We will also discuss previous pictures from the summer school

# Insurance data science : use and value of unusual data #1

Next week, with , I will be at the Summer School of the Swiss Association of Actuaries, in Lausanne, with Jean-Philippe Boucher (UQAM) and Ewen Gallic (AMSE).

I will give an introductionary talk on Monday morning, and the slides are now available

There will be some hands-on applications, on R. I will share some codes in the slides.

# SIDE Summer School, day 1

This morning, we start the SIdE (Italian Econometric Association) Summer School, on Machine Learning Algorithms for Econometricians. Emmanuel Flachaire will start with a presentation of nonparametric econometric techniques. I will then get back to the geometry of (standard) econometric techniques, to introduce kernels. The first series of slides are online.

I will then spend more time on the (popular) idea of “least squares” and mention other loss functions. Slides are online.

# What it the interpretation of the diagonal for a ROC curve

Last Friday, we discussed the use of ROC curves to describe the goodness of a classifier. I did say that I will post a brief paragraph on the interpretation of the diagonal. If you look around some say that it describes the “strategy of randomly guessing a class“, that it is obtained with “a diagnostic test that is no better than chance level“, even obtained by “making a prediction by tossing of an unbiased coin“.

Let us get back to ROC curves to illustrate those points. Consider a very simple dataset with 10 observations (that is not linearly separable)

x1 = c(.4,.55,.65,.9,.1,.35,.5,.15,.2,.85) x2 = c(.85,.95,.8,.87,.5,.55,.5,.2,.1,.3) y = c(1,1,1,1,1,0,0,1,0,0) df = data.frame(x1=x1,x2=x2,y=as.factor(y))

here we can check that, indeed, it is not separable

plot(x1,x2,col=c("red","blue")[1+y],pch=19)

Consider a logistic regression (the course is on linear models)

reg = glm(y~x1+x2,data=df,family=binomial(link = "logit"))

but any model here can be used… We can use our own function

Y=df$y S=predict(reg) roc.curve=function(s,print=FALSE){ Ps=(S&gt;=s)*1 FP=sum((Ps==1)*(Y==0))/sum(Y==0) TP=sum((Ps==1)*(Y==1))/sum(Y==1) if(print==TRUE){ print(table(Observed=Y,Predicted=Ps)) } vect=c(FP,TP) names(vect)=c("FPR","TPR") return(vect) } or any R package actually library(ROCR) perf=performance(prediction(S,Y),"tpr","fpr") We can plot the two simultaneously here plot(performance(prediction(S,Y),"tpr","fpr")) V=Vectorize(roc.curve)(seq(-5,5,length=251)) points(V[1,],V[2,]) segments(0,0,1,1,col="light blue") So our code works just fine, here. Let us consider various strategies that should lead us to the diagonal. The first one is : everyone has the same probability (say 50%) S=rep(.5,10) plot(performance(prediction(S,Y),"tpr","fpr")) V=Vectorize(roc.curve)(seq(0,1,length=251)) points(V[1,],V[2,]) Indeed, we have the diagonal. But to be honest, we have only two points here : $(0,0)$ and $(1,1)$. Claiming that we have a straight line is not very satisfying… Actually, note that we have this situation whatever the probability we choose S=rep(.2,10) plot(performance(prediction(S,Y),"tpr","fpr")) V=Vectorize(roc.curve)(seq(0,1,length=251)) points(V[1,],V[2,]) We can try another strategy, like “making a prediction by tossing of an unbiased coin“. This is what we obtain set.seed(1) S=sample(0:1,size=10,replace=TRUE) plot(performance(prediction(S,Y),"tpr","fpr")) V=Vectorize(roc.curve)(seq(0,1,length=251)) points(V[1,],V[2,]) segments(0,0,1,1,col="light blue") We can also try some sort of “random classifier”, where we choose the score randomly, say uniform on the unit interval set.seed(1) S=runif(10) plot(performance(prediction(S,Y),"tpr","fpr")) V=Vectorize(roc.curve)(seq(0,1,length=251)) points(V[1,],V[2,]) segments(0,0,1,1,col="light blue") Let us try to go further on that one. For convenience, let us consider another function to plot the ROC curve V=Vectorize(roc.curve)(seq(0,1,length=251)) roc_curve=Vectorize(function(x) max(V[2,which(V[1,]&lt;=x)])) We have the same line as previously x=seq(0,1,by=.025) y=roc_curve(x) lines(x,y,type="s",col="red") But now, consider many scoring strategies, all randomly chosen MY=matrix(NA,500,length(y)) for(i in 1:500){ S=runif(10) V=Vectorize(roc.curve)(seq(0,1,length=251)) MY[i,]=roc_curve(x) } plot(performance(prediction(S,df$y),"tpr","fpr"),col="white")  for(i in 1:500){  lines(x,MY[i,],col=rgb(0,0,1,.3),type="s") }  lines(c(0,x),c(0,apply(MY,2,mean)),col="red",type="s",lwd=3) segments(0,0,1,1,col="light blue")

The red line is the average of all random classifiers. It is not a straight line, be we observe oscillations around the diagonal.

Consider a dataset with more observations

 myocarde = read.table("http://freakonometrics.free.fr/myocarde.csv",head=TRUE, sep=";")  myocarde$PRONO = (myocarde$PRONO=="SURVIE")*1  reg = glm(PRONO~.,data=myocarde,family=binomial(link = "logit"))  Y=myocarde$PRONO S=predict(reg) plot(performance(prediction(S,Y),"tpr","fpr")) V=Vectorize(roc.curve)(seq(-5,5,length=251)) points(V[1,],V[2,]) segments(0,0,1,1,col="light blue") Here is a “random classifier” where we draw scores randomly on the unit interval S=runif(nrow(myocarde) plot(performance(prediction(S,Y),"tpr","fpr")) V=Vectorize(roc.curve)(seq(-5,5,length=251)) points(V[1,],V[2,]) segments(0,0,1,1,col="light blue") And if we do that 500 times, we obtain, on average MY=matrix(NA,500,length(y)) for(i in 1:500){ S=runif(length(Y)) V=Vectorize(roc.curve)(seq(0,1,length=251)) MY[i,]=roc_curve(x) } plot(performance(prediction(S,Y),"tpr","fpr"),col="white") for(i in 1:500){ lines(x,MY[i,],col=rgb(0,0,1,.3),type="s") } lines(c(0,x),c(0,apply(MY,2,mean)),col="red",type="s",lwd=3) segments(0,0,1,1,col="light blue") So, it looks like me might say that the diagonal is what we have, on average, when drawing randomly scores on the unit interval… I did mention that an interesting visual tool could be related to the use of the Kolmogorov Smirnov statistic on classifiers. We can plot the two empirical cumulative distribution functions of the scores, given the response $Y$ score=data.frame(yobs=Y, ypred=predict(reg,type="response")) f0=c(0,sort(score$ypred[score$yobs==0]),1) f1=c(0,sort(score$ypred[score$yobs==1]),1) plot(f0,(0:(length(f0)-1))/(length(f0)-1),col="red",type="s",lwd=2,xlim=0:1) lines(f1,(0:(length(f1)-1))/(length(f1)-1),col="blue",type="s",lwd=2) we can also look at the distribution of the score, with the histogram (or density estimates) S=score$ypred  hist(S[Y==0],col=rgb(1,0,0,.2),  probability=TRUE,breaks=(0:10)/10,border="white")  hist(S[Y==1],col=rgb(0,0,1,.2),  probability=TRUE,breaks=(0:10)/10,border="white",add=TRUE)  lines(density(S[Y==0]),col="red",lwd=2,xlim=c(0,1))  lines(density(S[Y==1]),col="blue",lwd=2)

The underlying idea is the following : we do have a “perfect classifier” (top left corner)

is the supports of the scores do not overlap

otherwise, we should have errors. That the case below

we in 10% of the cases, we might have misclassification

or even more missclassification, with overlapping supports

Now, we have the diagonal

when the two conditional distributions of the scores are identical

Of course, that only valid when $n$ is very large, otherwise, it is only what we observe on average….

# Exotic link functions for GLMs

In my previous post on GLMs, I discussed power link functions. But there are much more links that can be used :

• The square root link (for the Poisson model)

Consider some random variable $Y$ with mean $\mu$ and variance $\sigma^2$. Using Taylor’s expansion,$$g(Y)\sim g(\mu)+(Y-\mu)g'(\mu)+\frac{1}{2}(Y-\mu)^2g''(\mu)$$we can write$$\mathbb{E}[g(Y)]\sim g(\mu)+\frac{\sigma^2}{2}g''(\mu)$$ $$\text{Var}[g(Y)]\sim [g'(\mu)]^2\sigma^2$$

Assume that $Y\sim\mathcal{P}(\lambda)$, a consider a square root transformation, $g(y)=\sqrt{y}$, then the second equality becomes $$\text{Var}[\sqrt{Y}]\sim \left[\frac{1}{2\sqrt{\mathbb{E}[Y]}}\right]^2\text{Var}[Y]=\frac{1}{4}$$

So, somehow, with a square-root transformation, we have variance stability, which might be interpreted as some homoscedasticity.

• The complementary log-log function for the Bernoulli model

Assume that the true variable of interest is a Poisson one, $N|\mathbf{X}=\mathbf{x}\sim\mathcal{P}(\lambda_{\mathbf{x}})$ where $\lambda_{\mathbf{x}}=\exp[\mathbf{x}^T\mathbf{\beta}]$Thus,$$\mathbb{P}[N=0|\mathbf{X}=\mathbf{x}]=\exp[-\lambda_{\mathbf{x}}]=\exp[-(\exp[\mathbf{x}^T\mathbf{\beta}])]$$while$$\mathbb{P}[N>0|\mathbf{X}=\mathbf{x}]=1-\exp[-(\exp[\mathbf{x}^T\mathbf{\beta}])]=H(\mathbf{x}^T\mathbf{\beta})$$where $H(s)=1-\exp[-\exp(s)]$. Let $Y=\mathbf{1}(N>0)$. The previous model seems like a Bernoulli regression with $H$ as link function,$$\mathbb{P}[Y=1|\mathbf{X}=\mathbf{x}]=H(\mathbf{x}^T\mathbf{\beta})$$

So, assume now that instead of observing $N$ we observe $Y=\boldsymbol{1}(N>0)$. In that case, running a Bernoulli regression with a complementary log-log link function would be the same (?) as running first a Poisson regression on the original data, and then use it on our binary variable, zero vs. non-zero. Let us generate some data, and see what’s going on. Let us compare $e^{\lambda_{\mathbf{x}}}$ and $p_{\mathbf{x}}$ obtained from a standard logistic regression

n=563 set.seed(1) base=data.frame(X1=rnorm(n),X2=rnorm(n)) lambda=base$X1+base$X2 base$Y=rpois(n,exp(lambda)) regPois = glm(Y~.,data=base,family=poisson(link="log")) lambda = predict(regPois,type="response") regBinom = glm((Y==0)~.,data=base,family=binomial(link="probit")) prob = predict(regBinom, type="response") plot(prob,exp(-lambda),xlim=0:1,ylim=0:1) abline(a=0,b=1,lty=2,col="red") What if $p_{\mathbf{x}}$ was obtained from a Bernoulli regression, with a cloglog link function ? regBinom = glm((Y&gt;0)~.,data=base,family=binomial(link="cloglog")) prob = predict(regBinom, type="response") plot(prob,1-exp(-lambda),xlim=0:1,ylim=0:1) abline(a=0,b=1,lty=2,col="red") It looks like the fit is very good here ! Now, what if we have real data, like the dataset from A Theory of Extramarital Affairs, by Ray Fair, published in 1978 in the Journal of Political Economy (with 563 observations, and nine variables) base = read.table("http://freakonometrics.free.fr/baseaffairs.txt",header=TRUE) str(base) x=base$SEX base$SEX="M" base$SEX[x=="0"]="F" x=base$CHILDREN base$CHILDREN="YES" base\$CHILDREN[x==0]="NO" regPois = glm(Y~.,data=base,family=poisson(link="log")) lambda = predict(regPois,type="response") regBinom = glm((Y==0)~.,data=base,family=binomial(link="probit")) prob = predict(regBinom, type="response") plot(prob,exp(-lambda),xlim=0:1,ylim=0:1) abline(a=0,b=1,lty=2,col="red")

In that case the two models are very different. But actually, so is the second one

regBinom = glm((Y&gt;0)~.,data=base,family=binomial(link="cloglog")) prob = predict(regBinom, type="response") plot(prob,1-exp(-lambda),xlim=0:1,ylim=0:1) abline(a=0,b=1,lty=2,col="red")

How can we interpret that ? Could it be because the Poisson model is not good ? Actually, if we run a zero-inflated model here,

library(pscl) regZIP = zeroinfl(Y ~ . | ., data = base) summary(regZIP)   Count model coefficients (poisson with log link): Estimate Std. Error z value Pr(&gt;|z|) (Intercept) -0.002274 0.048413 -0.047 0.963 X1 1.019814 0.026186 38.945 &lt;2e-16 *** X2 1.004814 0.024172 41.570 &lt;2e-16 *** Zero-inflation model coefficients (binomial with logit link): Estimate Std. Error z value Pr(&gt;|z|) (Intercept) -4.90190 2.07846 -2.358 0.0184 * X1 -2.00227 0.86897 -2.304 0.0212 * X2 -0.01545 0.96121 -0.016 0.9872 --- Signif. codes: 0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1

Hence, we reject here the Poisson distribution assumption, because of the inflation of zeros… It looks like the cloglog link can be used to check if the Poisson distribution is a good model, or not…