Definition and Measurement of Selection Bias: From Constant Ratio to Constant Difference

Despite its intuitive appeal and popularity, Thorndike’s constant ratio (CR) model for unbiased selection is inherently inconsistent in n-free selection. Satisfaction of the condition for unbiased selection, when formulated in terms of success/acceptance probabilities, usually precludes satisfaction by the converse probabilities of rejection/failure, and vice versa. This paper suggests that this inconsistency is unavoidable due to the (negative) linear relation between “percent accepted” (P) and “percent rejected” (1 –P ), which does not preserve ratios and, thus, invalidates the conceptualization and measurement of selection bias in ratio terms. Therefore, we propose to substitute the CR model with a constant difference (CD) model for the definition and measurement of selection bias, and show the latter’s underlying rationale and its applicability in both n-free and fixed-n selection. The psychometric literature dealing with bias in selection has focused mainly on personnel and educational selection where the criterion of merit Y—a measure