Four-fifths rule
Also called 80% rule, adverse impact ratio
A screening test for adverse impact: if a group’s selection rate is less than four-fifths of the highest group’s selection rate, that is evidence of adverse impact requiring investigation.
Last reviewed
In practice
It originates in the US Uniform Guidelines on Employee Selection Procedures, published in 1978, and it has been the practical starting point for adverse-impact analysis ever since. The arithmetic is trivial: divide one selection rate by the other and compare to 0.8.
It is a heuristic, not a legal verdict and not a safe harbour. Regulators have been explicit that clearing the ratio does not establish that a practice is lawful, and courts consider statistical significance and practical significance alongside it.
Small numbers break it. In a group of four people, one selection swings the ratio wildly, which is why serious analysis sets a minimum cohort size before a category is analysed at all and a minimum group size before a result is flagged.
Related
Adverse impact
A substantially different rate of selection for a protected group that results from a neutral-seeming employment practice, regardless of whether anyone intended it.
Selection rate
The proportion of candidates from a given group who pass a particular stage of a hiring process.
Bias audit
An assessment of whether a hiring tool produces different selection rates across protected groups, performed by a party independent of the tool’s vendor.