1. for two tests (A and B) of the same hypothesis operating at the same significance level, the ratio of the number of cases needed by Test A to the number needed by Test B for each to have the same statistical power. The relative efficiency value enables a researcher to determine whether there is a preferred statistical approach for evaluating a particular phenomenon.

2. for two parameter estimates (A and B), a value reflecting the ratio of the standard error of Estimate A to the standard error of Estimate B.