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In a well-conducted, slightly idealized, randomized experiment, the only explanation of an association between treatment and outcome is an effect caused by the treatment. However, this is not true in observational studies of treatment effects, in which treatment and outcomes may be associated because of some bias in the assignment of treatments to individuals. When added to the design of an observational study, quasi-experimental devices investigate empirically a particular rival explanation or counterclaim, often attempting to preempt anticipated counterclaims. This review has three parts: a discussion of the often misunderstood logic of quasi-experimental devices; a brief overview of the important work of Donald T. Campbell and his colleagues (excellent expositions of this work have been published elsewhere); and its main topic, descriptions and empirical examples of newer devices, including evidence factors, differential effects, and the computerized construction of quasi-experiments.
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