Shortly after publication, Donald T. Campbell and Julian C. Stanley's "Experimental and Quasi-Experimental Designs for Research on Teaching" (1963) gained the status of a classic exposition of experimentation in education. There have been few attempts to extend this pioneering work, as might be expected of a work so comprehensive in conception and so brilliant in execution. Webb et al. (1966) produced a work similar in purposethat being to identify sources of external invalidity which arise from the reactive effect of measurement. We know of no other published work building on Campbell and Stanley's chapter.We feel that external validity was not treated as comprehensively as internal validity in the Campbell-Stanley chapter. Thus we have endeavored here to refine and elaborate on the sources of external invalidity identified by Campbell and Stanley and to propose and illustrate additional sources of external invalidity which merit attention.The intent (sometimes explicitly stated, sometimes not) of almost all experimenters is to generalize their findings to some group of subjects and set of conditions that are not included in 1 We would be remiss if we began without acknowledging a great debt to Donald T. Campbell and Julian C. Stanley for having inaugurated this discussion and, more personally, for having offered suggestions for improving an early draft of this manuscript. We also wish to thank Richard C. Anderson for many helpful suggestions.
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