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While a preexperimental study follows one group of participants receiving the intervention, a quasi-experimental design uses two or more study groups. The study groups are preexisting sets of people sharing some common variable. For example, participants already enrolled in a program may be compared with individuals waiting to receive the intervention, or with individuals who have chosen some other treatment or no treatment. The term "control group" is commonly used to describe the comparison group in a quasi-experimental design; however, some researchers do not consider quasi-experimental designs to have a true control condition because the study groups are preexisting or self-selected rather than created through random assignment.

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