To calculate a selection rate, divide the number of people from a group who pass a step by the number from that group who entered it. If 50 of 100 applicants in one group pass a screening step, their selection rate is 50%. Rates are worked out separately for each group and each step, because a process can look balanced overall while one stage treats groups very differently.
Comparing selection rates is the standard first check for adverse impact. Under the four-fifths rule, a group whose rate is below 80% of the highest group's rate is flagged for review. Small samples can make rates swing, so teams look at trends over time and investigate the criteria behind any gap, checking whether each one is genuinely needed for the job.