Reliability of Difference Scores Obtained From Nested Data Within a Multivariate Generalizability Theory Framework

The purpose of this study is to examine the reliability and dependability of difference scores computed as the change between a pretest and a posttest administered to assess the effectiveness of an intervention. The data-collection design involved a nested structure, with persons ( p ) nested within groups ( g ), and groups nested within sites ( s ). Multivariate generalizability theory was employed to estimate the reliability and dependability of difference scores at the levels of persons, groups, and sites. The G study designs included ( p • : g • : s • ) x i • , ( p • : s • ) x i • , ( p • : g • ) x i • , g • x i • , p • x i • , and s • x i • , with pretest and posttest serving as the two levels of the multivariate facet. In the design