Abstract
True experimental designs (Designs 4, 5, and 6 of Campbell & Stanley, 1963) are thought to provide internally valid results. This paper describes five studies involving the evaluation of various treat ment interventions and identifies a source of inter nal invalidity when self-report measures are used in a Pretest-Posttest manner. An alternative approach (Retrospective Pretest-Posttest design) to measuring change is suggested, and data comparing its ac curacy with the traditional Pretest-Posttest design in measuring treatment effects is presented. Finally, the implications of these findings for evaluation re search using self-report instruments and the strengths and limitations of retrospective measures are discussed.
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