  Ad platforms such as Facebook, Google and LinkedIn promise value for
advertisers through their targeted advertising. However, multiple studies have
shown that ad delivery on such platforms can be skewed by gender or race due to
hidden algorithmic optimization by the platforms, even when not requested by
the advertisers. Building on prior work measuring skew in ad delivery, we
develop a new methodology for black-box auditing of algorithms for
discrimination in the delivery of job advertisements. Our first contribution is
to identify the distinction between skew in ad delivery due to protected
categories such as gender or race, from skew due to differences in
qualification among people in the targeted audience. This distinction is
important in U.S. law, where ads may be targeted based on qualifications, but
not on protected categories. Second, we develop an auditing methodology that
distinguishes between skew explainable by differences in qualifications from
other factors, such as the ad platform's optimization for engagement or
training its algorithms on biased data. Our method controls for job
qualification by comparing ad delivery of two concurrent ads for similar jobs,
but for a pair of companies with different de facto gender distributions of
employees. We describe the careful statistical tests that establish evidence of
non-qualification skew in the results. Third, we apply our proposed methodology
to two prominent targeted advertising platforms for job ads: Facebook and
LinkedIn. We confirm skew by gender in ad delivery on Facebook, and show that
it cannot be justified by differences in qualifications. We fail to find skew
in ad delivery on LinkedIn. Finally, we suggest improvements to ad platform
practices that could make external auditing of their algorithms in the public
interest more feasible and accurate.
