Estimation of the Reliability of Differences Under Revised Reliabilities of Component Scores

Projecting the changes in the reliability of a difference score (d =× ‐ Y ) as a consequence of changes in the reliabilities of X and Y does not represent a straightforward application of the Spearman‐Brown formula. Formulas are developed for estimating the changes in the reliability of X‐Y under two possible assumptions: (a ) × and Y have equal variances both before and after their reliabilities are altered, and (b ) × and Y have unequal variances before and after × and Y are modified. The second of these situations, which includes the first as a special case, is probably the more common .