The usual way teams check a resume screener for bias is retrospective: pull last quarter's outcomes, group candidates by demographic, and compare selection rates. That is the four-fifths rule in practice. The EEOC's Uniform Guidelines treat a selection rate for any group below 80% of the highest group's rate as evidence of adverse impact, and analysts compute it by dividing each group's selection count by its applicant count (Berkshire Associates; BambooHR). The check is worth running, but it has a structural blind spot: it needs outcomes that already happened, it needs demographic labels you may not hold cleanly, and it cannot separate a school-name preference from a dozen other correlations in the pool.
Adverse impact is a statistical signal, not proof of intent, and the legal theory it triggers is disparate impact, which needs no discriminatory motive at all (Pin). A retrospective ratio tells you something is off somewhere in the funnel. It rarely tells you which field caused it.
A controlled probe inverts the direction of the investigation. Instead of asking "did group A fare worse than group B," you ask "if the only thing that changed is the school name, does the score change?" That is an experiment, and experiments isolate cause. The University of Washington study that made this idea famous varied candidate names across hundreds of real resumes and over three million job comparisons, and found the models favored white-associated names 85% of the time and Black-associated names 9% of the time (UW News; Brookings). The design is the lesson: change one attribute, hold everything else constant, measure the ranking.
The same design pattern underpins older selection research. Structured interviews that score every candidate against the same written criteria predict job performance far better than unstructured ones, and the reason is that fixed criteria remove room for an evaluator to drift (Pin's scorecard guide). A screener is an evaluator. A planted-variation test is how you find out whether its criteria include a field they should not.



