The 4/5ths Rule: What It Measures and What It Misses

The 4/5ths rule, also called the 80% rule, is the threshold established by the Uniform Guidelines on Employee Selection Procedures (UGESP) for identifying potential adverse impact in hiring. It is widely cited and frequently misunderstood.

Understanding what it actually measures, and where it does not apply, is essential for building a monitoring process that holds up under scrutiny.

How the calculation works

The 4/5ths rule compares selection rates across demographic groups. The selection rate for any group should be at least 4/5ths (80%) of the selection rate for the group with the highest selection rate.

The calculation:

1. Identify the group with the highest selection rate. Call this the reference group.
2. Calculate the selection rate for each comparison group (number selected / number who applied).
3. Divide each comparison group’s rate by the reference group’s rate.
4. If the result is below 0.80 for any group, adverse impact is indicated.

Example:

• 50 white applicants, 40 selected: selection rate = 80%
• 30 Black applicants, 18 selected: selection rate = 60%
• 60% / 80% = 0.75

0.75 is below 0.80, so adverse impact is indicated for Black applicants in this example.

What the 4/5ths rule does not measure

Statistical significance. The 4/5ths rule is a practical threshold, not a statistical test. With small sample sizes, a ratio below 0.80 may result from random variation rather than a systematic problem. Courts and regulators have increasingly required statistical significance testing (typically a two-standard-deviation test) alongside the 4/5ths rule.

Intersectionality. The rule is calculated separately for each demographic group. It does not capture disparities that occur at the intersection of race and gender, for example. A process may pass the 4/5ths rule for women and separately for Black candidates while producing significant adverse impact for Black women.

Disparate treatment. The 4/5ths rule measures outcomes, not intent. A process can produce equal outcomes while still involving discriminatory intent. Disparate treatment is a separate legal theory that requires different evidence.

What happens when adverse impact is indicated

Under UGESP, an employer who finds adverse impact must either:

1. Demonstrate that the selection procedure is valid and job-related, or
2. Modify or replace the selection procedure to reduce or eliminate the adverse impact

Validity is established through evidence: criterion-related validity studies, content validity documentation, or construct validity evidence. The burden of proof shifts to the employer once adverse impact is demonstrated.

How often to run the analysis

Adverse impact monitoring is not a one-time exercise. It should be conducted at each stage of the selection process (screening, assessment, interview, offer) and at regular intervals, typically annually.

Running it at each stage is important because a process can show no overall adverse impact while producing significant disparity at an intermediate step. If a résumé screening process eliminates a disproportionate number of qualified candidates from a protected group, that disparity is not visible in the final hire data alone.