What is the Solomon four-group design?

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The Solomon four-group design is a type of experiment where participants get randomly assigned to either 1 of 4 groups that differ in whether the participants receive the treatment and whether the outcome of interest is measured once or twice in each group. The four groups in this design are A treatment group with pre-intervention and post-intervention measurements (a.k.a. pretest and posttest). A control group with both pretest and posttest measurements. A treatment group with only a posttest measurement. And a control group with only a posttest measurement.