Studying cognitive development in retarded and nonretarded groups: the role of theory.

Stephens and McLaughlin's (1974) report of retarded-nonretarded group differences in performance on Piagetian reasoning tasks was critically examined. The findings were difficult to interpret in relation to major theories of cognition in retarded and nonretarded persons because of problems in the investigators' testing and subject selection procedures and because of an inappropriate use of analysis of covariance. These problems were discussed, their potential impact on the findings examined, and certain guidelines proposed for research of this genre--guidelines designed to sharpen the relationship between experimentation and theory.