Diferencia entre revisiones de «Fairness and bias correction in Machine Learning»

Revisión de 16:18 10 dic 2019

Version in Spanish: Equidad y corrección de sesgos en Aprendizaje Automático

Work in progress.

Criteria in Classification problems^[1]

In classification an algorithm learns a function to predict a discrete characteristic Y, the target variable, from known characteristics X. We model A as a discrete wikipedia:random variable which encodes some characteristics contained or implictly encoded in X that we consider as sensitive characteristics (gender, ethnicity, sexuality, etc.). We finally denote by R the prediction of the classifier.

Independence

• The variables <math>(R,A)</math> satisfy independence if <math> R \bot A </math>.

Independence requires the sensitive characteristics to be statistically independent to the prediction. Another way to express this is

<math> \mathbb{P}(R = r | A = a) = \mathbb{P}(R = r | A = b) \quad \forall r \in R \quad \forall a,b \in A </math>

This means that the probability of being classified by the algorithm in each of the groups equal for two individuals with different sensitive characteristics.

Yet another equivalent expression for this is to use the concept of wikipedia:mutual information between wikipedia:random variables defined as <math> I(X,Y) = H(X) + H(Y) - H(X,Y) </math> where H is the wikipedia:entropy of the wikipedia:random variable. Then <math> (R,A) </math> satisfy independence if <math> I(R,A) = 0 </math>.

Possible relaxations include introducing a positive slack <math> \epsilon > 0 </math> and require

<math> \mathbb{P}(R = r | A = a) \geq \mathbb{P}(R = r | A = b) - \epsilon \quad \forall r \in R \quad \forall a,b \in A </math>

Another possible relaxation is to require <math> I(R,A) \leq \epsilon </math>

Separation

• The variables <math>(R,A,Y)</math> satisfy separation if <math> R \bot A | Y </math>.

Separation requires the sensitive characteristics to be statistically independent to the prediction given the target variable. Another way to express this is

<math> \mathbb{P}(R = r | Y = q, A = a) = \mathbb{P}(R = r | Y = q, A = b) \quad \forall r \in R \quad q \in Y \ quad \forall a,b \in A </math>

This means that the probability of being classified by the algorithm in each of the groups is equal for two individuals with different sensitive characteristics given that they actually belong in the same group (have the same target variable).

Another equivalent expression, in the case of a binary target rate, is that the true positive rate and the false positive rate are equal (and therefore the false negative rate and the true negative rateare equal) for every value of the sensitive characteristics:

<math> \mathbb{P}(R = 1 | Y = 1, A = a) = \mathbb{P}(R = 1 | Y = 1, A = b) \ quad \forall a,b \in A </math>

<math> \mathbb{P}(R = 1 | Y = 0, A = a) = \mathbb{P}(R = 1 | Y = 0 , A = b) \ quad \forall a,b \in A </math>

One possible relaxation of this condition is that the difference between rates is not zero but rather a positive slack <math> \epsilon > 0 </math>.

Sufficency

• The variables <math>(R,A,Y)</math> satisfy separation if <math> Y \bot A | R </math>.

Sufficency requires the sensitive characteristics to be statistically independent to the target variable given the prediction. Another way to express this is

<math> \mathbb{P}(Y = q | R = r, A = a) = \mathbb{P}(Y = q | R = r, A = b) \quad \forall r \in R \quad q \in Y \ quad \forall a,b \in A </math>

This means that the probability of actually being in each of the groups is equal for two individuals with different sensitive characteristics given that you they have been predicted to belong to the same group.

Relationships between definitions

Here we sum up some of the main results that relate the definitions from this section:

• If A and Y are not statistically independent, then sufficency and independence cannot both hold.
• If A and Y are not statistically independent, Y is binary, and R and Y are not statistically independent. Then independence and separation cannot both hold.
• If <math> (A,R,Y) </math> as a wikipedia:joint distribution has nonzero [[wikipedia:probability] for all its possible values and A and Y are not statistically independent, then separation and sufficency cannot both hold.

Other criteria

The following criteria can be understood as measures of this three definitions or a relaxation of them. In Figure 1 we can see the relationships between them.

Metrics^[2]

Most statistical measures of fairness rely on different metrics, so we will start by defining them. When working with a binary classifier, both the predicted and the actual classes can take two values: positive and negative. Now let us start explaining the different possible relations between predicted and actual outcome:

• True positive (TP): The case where both the predicted and the actual outcome are in the positive class.
• True negative (TN): The case where both the predicted and the actual outcome are in the negative class.
• False positive (FP): A case predicted to be in the positive class when the actual outcome is in the negative one.
• False negative (FN): A case predicted to be in the negative class when the actual outcome is in the positive one.

References

1. ↑ Solon Barocas,Moritz Hardt,Arvind Narayanan. Fairness and Machine Learning. http://www.fairmlbook.org, 2019.
2. ↑ Sahil Verma; Julia Rubin, Fairness Definitions Explained, (IEEE/ACM International Workshop on Software Fairness, 2018).