Restriction of range occurs when a variable has less variability in a study sample than in the full population. The restricted range can be present for observed variables (dependent or independent variables) or in other variables that were not measured. Restricted range can affect statistical inferences, typically in the direction of underestimating the effect sizes and underestimating validity coefficients of associations between predictors and outcomes.

For example, consider the scenario in which scores from a college entrance examination, such as the SAT (x variable), are used to predict first-year college grade point average or GPA (y variable). Many colleges base their admission criteria on SAT scores, and therefore first-year college GPA data would only be available for a restricted sample (i.e., students with SAT scores above ...