Use and Misuse of No-Difference Findings in Eliminating Threats To Validity

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Abstract
Donald Campbell and his colleagues have elaborated an approach to research based on the elimination of threats to four validity types: internal, external, construct, and statistical conclusion. The central thesis of this article is that the process of eliminating validity threats depends fundamentally on no-difference findings, a fact that, unfor tunately, has not been made explicit by researchers. The implications of this neglect are explored using examples from a number of different substantive areas such as psychology, health, and medicine. Finally, the intrinsic role of no-difference findings is described in the context of all four validity types, and suggestions for improving the process are offered.