Quasi Experiments

Margaret H. Clark & William R. Shadish

Quasi experiments, like all experiments, manipulate treatments to discover causal effects (quasi experiments are sometimes referred to as nonrandomized experiments or observational studies). However, these experiments differ from randomized experiments in that units are not randomly assigned to conditions. Quasi experiments are often used when it is not possible to randomize ethically or feasibly. Therefore, units may be assigned to conditions using a variety of nonrandomized techniques, such as permitting units to self-select into conditions or assigning them based on need or some other criterion. Unfortunately, quasi experiments may not yield the unbiased estimates that randomized experiments yield because quasi experiments can neither reliably rule out alternative explanations for the effects nor create error terms that are orthogonal to treatment. To improve causal inferences in quasi experiments, however, researchers can use a combination of design features, practical logic, and statistical analysis. Although researchers had been using quasi-experimental designs long before ...

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