Four-fifths rule
The four-fifths rule is a screening heuristic for adverse impact: if one group's selection rate is below 80% of the highest group's rate, the practice warrants investigation. It originates in US EEOC guidance and is widely used as an early warning signal.
It is a diagnostic prompt rather than a legal test. Failing it does not establish discrimination, and passing it does not establish its absence — particularly with small sample sizes, where the ratio is statistically unstable.
Its usefulness is that it forces measurement. Most organisations do not calculate selection rates by group at all.