Background

This is a walk-through explanation of the Symbolic Volume Integration method mentioned in Part I of this Algorithimic Fairness post series.

In order to prove or disprove the fairness of an algorithm, we must prove or disprove the following inequality:

where $$v_s$$ is an entry of $$\vec{v}$$ indicating whether the person belongs to a protected class—e.g., a particular religion or ethnicity—and $$\epsilon < 1$$ is some agreed-upon or mandated standard of fairness, with smaller $$\epsilon$$s implying stricter fairness requirements.

Motivation: Proof Beats Confidence

In the world of scientific computing, probabilities—like those in the above inequality—are typically estimated using Markov Chain Monte Carlo techniques, which are known to perform well in a variety of cases and allow the underlying process to be treated as a black box. Due to the stochastic nature of MCMC, any conclusion based on it must be stated in terms of statistical certainty. In this respect, the new Symbolic Volume Integration method has an advantage over MCMC—it allows a proof of the fairness condition, rather than a statement of statistical confidence. With proof within convenient reach, it is unjustifiable to settle for mere confidence.

Symbolic Volume Integration can prove (or disprove) the fairness inequality because it guarantees a lower bound on its integral. Note that in a probabilistic setting the method can also be used to compute upper bounds on probabilities; if we need an upper bound on $$P[A] = 1 - P[\neg A]$$, it suffices to find a lower bound on $$P[\neg A]$$. By finding lower bounds on the probabilities in the numerator and upper bounds on the probabilities in the denominator, we can prove the fairness condition. Likewise, upper bounds on the numerator and lower bounds on the denominator would allow us to disprove the fairness condition.

While Symbolic Volume Integration allows us to obtain a proof of algorithmic fairness, it does require an intimate knowledge of the algorithm in question. A black box representation of the algorithm will not suffice; Symbolic Volume Integration requires the contents of the algorithm so that it can translate them into a set of logical constraints1.

The Algorithm

Symbolic Volume Integration consists of the following steps:

• Translate Code into a Predicate Logic Formula
• Assuming that the population model and decision program can both be expressed as sequences of assignments, probabilistic assignments, and conditional expressions involving simple arithmetic (addition and multiplication), we can translate their sequential, imperative commands into conjunctions of declarative logical statements.
• Since we ultimately care about the composition of the population model and the decision program—letting the outputs of the population model be the inputs of the decision program—we can create one big logic formula by conjoining their individual logic formulas.
• We conjoin additional requirements to the formula, in order to specify the possibility for which we wish to compute probability. For example, if we wanted to compute $$P[\text{hired} = true \land \text{age} \ge 60]$$ , we would conjoin $$(\text{hired} = true \land \text{age} \ge 60)$$ to the formula. We now have a Big Formula.
• For each probabilistic assignment in the program, you can think of there being a free variable in our big formula. E.g. if a random value for “age” is generated in the population model, it would be a free variable in the logic formula.
• The free variables just mentioned form a real space, $$\mathbb{R}^n$$ (where $$n$$ is the number of free variables). Only certain combinations of these variables will actually satisfy the big logic formula we constructed. It turns out that the valid combinations form a region in $$\mathbb{R}^n$$, which we will call the “admissible region”. We can find points in the admissible region by using a SMT solver (“Satisfiability Modulo Theories”; in our case, we use an SMT solver called Z3). Bear in mind that the free variables are probabilistically assigned, so each of them has an associated PDF. In combination, this implies a joint PDF over $$\mathbb{R}^n$$ formed by their product (they are independently distributed). Hence, computing probability for a given scenario entails integration of the joint PDF over the scenario’s admissible region.
• Decompose the Admissible Region into Hyperrectangles
• Integrate over the Hyperrectangles and
1. Proving an algorithm’s fairness would in practice require access to the algorithm’s source and parameters. Perhaps future regulators would use black box MCMC methods to identify possible violations of fairness and then request the necessary details as part of an audit.