Tag Archives: calibration

“Scope and limits of artificial intelligence” at the SCOR foundation monthly webinar

This morning, I will give a talk on “scope and limits of artificial intelligence” at the SCOR foundation monthly webinar. As discussed previsously, we currently have ongoing research on discrimination and fairness founded by the fondation (newsletter #1 is online).

Insurance (and further motivations)

Since we will talk about fairness, I will start with a couple of motivations. The first one is about COMPAS,

Interestingly, we have the data to analyse that one. In the original analysis, conditional on non-re-offending, proportions of being wrongly classified in the two protected groups are significantly different, so the algorithm is racist,

The answer was that actually, conditional on being classified as high risk, the probability of re-offense in the two protected groups are significantly similar, so the algorithm is not racist,

So clearly, we can start to see that it will not be so easy, since using the same data and the same models, two different conclusions can be obtained.

We will also disccuss legal aspects.

This idea of “determining actuarial factor” has been remove in Europe, but we can still find it in Québec

I can also mention some recent projects, in Colorado, where insurers are asked  to predict race and ethnicity (that specific topic is on our agenda for the summer)

And finally, I should stress that discrimination has not much to do with the intention of the statistician. This is the idea of indirect discrimination

I should also mention “redlining“. About 100 years ago, in the US, we started to see maps, created by HOLC (based on City Survey Files, 1935-1940). Those maps contained “red” areas and “green” areas. Bankers were supposed to avoid the red areas, because they were considered too risky.

As a sidenote, we see nowadays some blue-lining related to climate risks,

“Blue-lining,” from the consumer’s perspective, is when banks or mortgage lenders draw lines of risk around certain streets or neighborhoods, often without clear disclosure.

Finally, I just want to recall that algorithms just tend to reproduce what can be observed in data. If there is a difference between men and women, they will reproduce it.

A bit more on insurance

I should also stress an important problem (that could be related to a paper we wrote, in French, a few years ago). Classically, when modeling categorical variables, such as a binary variable y\in\{0,1\}, practitionners just care about getting the good category. On the left, we have pictures of cats and dogs to train a model, then we try on a new picture, that is either a cat or a dog. Somehow, there is a ground truth and it is possible to see if we are right or wrong. Same if we want to detect a disease on medical pictures. Now, if we move to the right. In the middle, we have a model that predicts if it will rain, or not. But here, maybe, what we care about is actually the probability to have rain. On the right, we have the actuarial problem of modeling claims frequencies. We do not want to predict who will claim a loss, but we want a good estimator of the probability to claim a loss. The challenge, clearly, is that we cannot observe that one. We cannot observe the latent risk factor. We only observe if people got an accident or not. But some people with a very small probability can still claim a loss. And very bad drivers can actually be very lucky, and got no accident one year.

Again, in insurance, we care more about the score, the estimation of the probability than the class \widehat{y}. So we can slightly modify standard fairness definitions, to be based not on predicted classes \widehat{y}, but on the score m(\boldsymbol{x},s). As we will discuss, there are usually three general definitions of so-called “group fairness”

Quantifying unfairness with optimal transport

Let us start with demographic parity. A weak version is that, on average, scores in the two groups should identical (or close). An alternative is the strong version, asking for equalities in distributions : for any set \mathcal{I}\subset[0,1], the probability that the score is in \mathcal{I} (e.g. between 40% and 60%) should be the same in the two groups.

Mathematically, we need a distance between the distributions of scores in the two groups. And a popular distance is Wasserstein distance, that is related to optimal transport.

The empirical version is perhaps easier to understand, and mapping is based on matching of individuals. xxx

As a cultural sidenote, a couple of slides to explain why it has to do with “optimal transport”, going back to Monge (1781)‘s problem. It’s all about transporting the sand, grain by grain, from the hole to the pile. Below, we have a (purely) random transport. Which is not efficient at all…

and then the optimal version (for a strictly convex cost function), he leftmost grain in the hole goes on the lefttmost part of the stack, etc.

Mitigation

For mitigation (once we have observe that there was discrimination, as discussed previously) heuristically, we want to be somewhere in between the two distributions in the two subgroups,

Being “in between” can be interpreted locally: for someone in group A, it should be between (weights are related to proportions in the two groups) the prediction, as someone in group A, and then some sort of counterfactual in the other group, namely the prediction that person would have obtained if she had been in group B, based on the same probability level,

For the other group it is the opposite

Beyond demographic parity

If we get back to our COMPAS examples, demographic parity, in the standard classification-based definition, would be translated as

If we get back to the original motivation we gave, it had nothing to do with demographic parity, the first slide had to do with separation, or equalized odds, while the second one had to do with sufficiency, or calibration.

More generally, if we consider a weak version of the independence criterias, we have moments equality, within each protected subgroup,

Let us mention a bit more calibration. Calibration is deeply related to the interpretation of “probabilities” as returned by models as “real probabilities”. In machine learning, it is hard to define properly what those “probabilities” are.

Calibration is related to the following idea, discussed above: if we consider all cases where the predicted probability was 40% (or say, close to 40%), then the proportion on 1’s should be close to 40%.

To conclude that disgression, I can mention the following example highlighting that we should be concerned by probabilities returned by machine learning algorithms. Consider some pictures, generated by some algorithm, and more precisely, some flow of pictures, from a woman to a man

Below, we can see probabilities given by some online appplication, that returns probabilities to be a woman, given a picture. Can’t we agree that it is surprising that those probabilities (of beeing a woman) do not decrease continuous, from the picture in the top left corner and the one in the bottom right one ?

Finally, I can also mention “individual fairness”, or “counterfactual fairness”. Here also, optimal transport can be used, to quantify counterfactual unfairness. But I won’t be too long here.

Finally, an opening for next year’s agenda, with interpretability. Interpretability is a very important issue in actuarial science, which is not as objective as people might think, and the popular

let the data speak for itself

In insurance, interpretation is very important, probably more important than model assumptions

Interpretation become a key concept when dealing with multiple sensitive attributes

To conclude, just a final reminder that dealing with mitigation is a complex philosophical problem….

Tomorrow, we will discuss further at our workshop, in Québec city

Exposé au séminaire de statistique (StatQAM)

Tomorrow, Ewen Gallic will present some recent work at the StatQAM statistical seminar, on calibration, with Agathe Fernandes Machado, François Hu, and Emmanuel Flachaire. It will substantially be based on our recent paper From Uncertainty to Precision: Enhancing Binary Classifier Performance through Calibration

The assessment of binary classifier performance traditionally centers on discriminative ability using metrics, such as accuracy. However, these metrics often disregard the model’s inherent uncertainty, especially when dealing with sensitive decision-making domains, such as finance or healthcare. Given that model-predicted scores are commonly seen as event probabilities, calibration is crucial for accurate interpretation. In our study, we analyze the sensitivity of various calibration measures to score distortions and introduce a refined metric, the Local Calibration Score. Comparing recalibration methods, we advocate for local regressions, emphasizing their dual role as effective recalibration tools and facilitators of smoother visualizations. We apply these findings in a real-world scenario using Random Forest classifier and regressor to predict credit default while simultaneously measuring calibration during performance optimization.

To illustrate, consider predictions about the gender of the person on the picture, including probabilities (confidence), obtained from https://www.picpurify.com/demo-face-gender-age.html, with fake pictures, from https://www.nytimes.com/interactive/2020/11/21/science/artificial-intelligence-fake-people-faces.html.

From Uncertainty to Precision: Enhancing Binary Classifier Performance through Calibration

Our paper From Uncertainty to Precision: Enhancing Binary Classifier Performance through Calibration, written with Agathe Fernandes Machadoa, Emmanuel Flachaire, Ewen Gallic and François Hu is now online on ArXiv,

The assessment of binary classifier performance traditionally centers on discriminative ability using metrics, such as accuracy. However, these metrics often disregard the model’s inherent uncertainty, especially when dealing with sensitive decision-making domains, such as finance or healthcare. Given that model-predicted scores are commonly seen as event probabilities, calibration is crucial for accurate interpretation. In our study, we analyze the sensitivity of various calibration measures to score distortions and introduce a refined metric, the Local Calibration Score. Comparing recalibration methods, we advocate for local regressions, emphasizing their dual role as effective recalibration tools and facilitators of smoother visualizations. We apply these findings in a real-world scenario using Random Forest classifier and regressor to predict credit default while simultaneously measuring calibration during performance optimization.

Fairness and discrimination, PhD Course, #5 Models and Data

For the fifth course, we will discuss machine learning and standard techniques used to get predictive models, and to assess accuracy of those models.

GLM (possibly constrained)

Classically, we use a penalized version of least squares (but this can be adapted to GLMs, when penalizing the negative log-likelihood).  Because of Karush–Kuhn–Tucker conditions, having a constraint on the parameter is equivalent to the following penalized problem, when the constraint is on the \ell_2 norm of \boldsymbol{\beta},

We can also consider the \ell_1 norm of \boldsymbol{\beta},

Those two approaches can be see as a trade-off between accuracy (here the empirical risk on the left) and complexity of the model (on the right). And we can also consider a mixture of the two norms,

As we will see, it will also be possible to consider some penality related to fairness and discriminiation measures (in-processing).

Classifier and ROC Curves

We will also recall metrics used in the context of classification, such as the ROC curve

Each point of the curve can be related to two areas related to the distributions of the scores (in the two groups), for the same threshold – namely the false positive rate and true positive rate

Based on the ROC curve, we can define the AUC, the area under the ROC curve,

But for classifiers, the important challenge is to have calibrated scores, meaning that we want the score to be interpreted as the true underlying probability.

Calibration

Well-calibration is defined as follows

or (with different notations)

It is a well know properties in several applications.

https://freakonometrics.hypotheses.org/files/2024/01/cours-slides-fairness-07.png

The plot on the right is the calibration plot,

https://freakonometrics.hypotheses.org/files/2024/01/cours-slides-fairness-10.png

We can easily get that plot

https://freakonometrics.hypotheses.org/files/2024/01/cours-slides-fairness-09.png

This concept is related to the question “do probabilities returned by some model represent reals probabilities ?” For instance, below, we have pictures generated as some sort of geodesic between two pictures, with a woman on the top left, and a man in the bottom right, published in the New York Times. And below, “probabilities” given by  https://www.picpurify.com/demo-face-gender-age.html.

We could agree that it is rather strange that probabilities (to have a man) do not increase continuously, but on top, with extremely high confidence, the model predicts that the picture is the one of a woman, and below, also with extremely high confidence, that the person is a man…

Data, observations vs. experiments

Then, after concept and notations related to models, we will talk about data. More specifically, the distinction between observations and experimentations.

Another popular classification is the one discussed by Judea Pearl.

So we will talk about association, correlation, causal inference, and counterfactuals.

“Correlated variables” or proxies

One important issue, is that with massive data, one can easily get a (good) proxy of almost any sensitive variable.

The concept is related to comonotonicity, or perfect correlation.

But this is clearly too strong, so we will discuss depedence measures, too.

Independence properties

Recall that independence is defined as follows

https://freakonometrics.hypotheses.org/files/2024/01/cours-slides-fairness-11.png

and we can consider a weaker form, based on null-covariance

https://freakonometrics.hypotheses.org/files/2024/01/cours-slides-fairness-12.png

or null-correlation

(sidenote, this correlation measure is bounded, and those bounds are related to Hardy-Littlewood inequality and optimal transport)

An interesting measure is the maximal correlation

https://freakonometrics.hypotheses.org/files/2024/01/cours-slides-fairness-13.png

or we can consider a weaker version, without consider all possible transformation, but only a subset

https://freakonometrics.hypotheses.org/files/2024/01/cours-slides-fairness-14.png

Another important concept is the one of conditional independence

https://freakonometrics.hypotheses.org/files/2024/01/cours-slides-fairness-16.png

(the later will be used in the context of causal graphs).

Causality

Before talking about causality, recall that what non-independence mean…

https://freakonometrics.hypotheses.org/files/2024/01/cours-slides-fairness-17.png

We can then construct causal graphs, or “directed acyclic graphs”

https://freakonometrics.hypotheses.org/files/2024/01/cours-slides-fairness-20.png

where nodes are the variables used in the model, and the outcome (usually that the end of the causal graph). Then we define paths

https://freakonometrics.hypotheses.org/files/2024/01/cours-slides-fairness-18.png

and the concept of d-separation

https://freakonometrics.hypotheses.org/files/2024/01/cours-slides-fairness-19.png

This concept is related to the statistical property of conditional independence

https://freakonometrics.hypotheses.org/files/2024/01/cours-slides-fairness-21.png

More precisely, we have the following Markov property on causal graphs

https://freakonometrics.hypotheses.org/files/2024/01/cours-slides-fairness-22.png

For example, for such a graphical model,

the joint distribution is \mathbb{P}[x_1,x_2,x_3,x_4]=\mathbb{P}[x_1]\times \mathbb{P}[x_2|x_1]\times \mathbb{P}[x_3|x_2]\times \mathbb{P}[x_4|x_3]and for the graphical model below

we have\mathbb{P}[x_1,x_2,x_3,x_4]=\mathbb{P}[x_1]\times \mathbb{P}[x_2]\times \mathbb{P}[x_3|x_1,x_2]\times \mathbb{P}[x_4|x_3]Those graphs can be related to structural models (with idiosyncratic noise denoted U), since

https://freakonometrics.hypotheses.org/files/2024/01/cours-slides-fairness-23.png

Potential outome

Another important concept is the concept of counterfactuals, and potential outome. In an ideal world, we would have observed the outome in both cases, with and without the treatement

https://freakonometrics.hypotheses.org/files/2024/01/cours-slides-fairness-24.png

but in real life, it’s only one of them,

https://freakonometrics.hypotheses.org/files/2024/01/cours-slides-fairness-25.png

And the goal will be, somehow, to estimate what the non-observed outcome would be. And then, classical quantites we wish to estimate are the average treatement effect, and the conditional version, based on some covariates.

https://freakonometrics.hypotheses.org/files/2024/01/cours-slides-fairness-26.png

This concept will be related to counterfactual fairness actually, when the “treatement” will be the sensitive attribute.

Twin network representation of the counterfactual

Finally, we will consider a so-called “twin network representation”. Consider a DAG, associated with some simple structural model

https://freakonometrics.hypotheses.org/files/2024/01/cours-slides-fairness-27.png

Based on a structural model, we can get values of idiosyncratic noise component

https://freakonometrics.hypotheses.org/files/2024/01/cours-slides-fairness-28.png

Then, we use those values on the twin representation, when the treatement is not 0, but 1. Counterfactuals are created by using the same noises

https://freakonometrics.hypotheses.org/files/2024/01/cours-slides-fairness-29.png

The difference between the two outcomes is the treatement effect, or the disparate treatement

https://freakonometrics.hypotheses.org/files/2024/01/cours-slides-fairness-30.png

or more generally, we write

https://freakonometrics.hypotheses.org/files/2024/01/cours-slides-fairness-31.png

This is an idea used in Plecko & Meinshausen, 2019, in the context of fairness, but we will discuss this more, later on…