Pre-experimental designs lack any equivalence of groups and include one- and two-group ex post facto and one-group before-after designs.

Question

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Pre-experimental designs do not involve random assignment to groups, which means there is no control over external variables that could influence the outcome, leading to a lack of equivalence between groups. These designs include one- and two-group ex post facto studies, where researchers examine the effect of an independent variable on a dependent variable without manipulating the independent variable, and one-group before-after designs, where the same group is measured before and after an intervention, without a comparison group.

Pre-Experimental Designs Lack Any Equivalence of Groups and Include One-