A quasi-experimental approach to assessing treatment effects in the nonequivalent control group design.

Abstract

Four statistical tests of treatment effect are evaluated for the nonequivalent control group design. This design consists of pre- and posttreatment measures of a dependent variable with biased assignment to treatment groups. The biased assignment creates a treatment-pretest confounding for which different statistical techniques adjust. The different statistical tests discussed are the analysis of covariance, analysis of covariance with reliability correction, raw change score analysis, and standardized change score analysis. If assignment to treatment groups is based on -j^ ,the pretest score (a very infrequent event), analysis of covariance is the appropriate mode of analysis. Selection based on the pretest true scores necessitates a reliability correction procedure. Selection based on stable group differences and selection that -occurs midway between the pre- and posttest necessitates change score analysis. Compensatory education programs can be made to look mistakenly harmful if they are analyzed by an inappropriate statistical technique when they should have been analyzed by standardized change score analysis. Given a model of the process of ^selection into treatment groups, the nonequivalent control group design can yield Snterpretable results.