In a pretest-postest design, a sample is randomly assigned to two or more groups (usually one or more treatment groups and one control group); Subjects in each group are measured at two time periods: pretest (before treatment) and posttest (after treatment). Subjects in the same group receive the same treatment. The treatment, especially for a control group, may be a placebo or no treatment at all. Show
We consider multiple designs for the simple case of a single treatment group and a single control group, as shown in Figure 1. Figure 1 – Pretest-Posttest Data T-test of the GainOne approach is to simply compare the gain after treatment for the two groups. Here the gain is the difference between the posttest and pretest scores. This can be done by using a two-sample t-test based on the data in range E4:E8 vs. E9:E13, as shown in Figure 2. Figure 2 – t-test on the gain With p-value = .003, we see there is a significant difference between the Treatment and Control groups. Note that the study as a whole depends on the fact that there isn’t a significant difference between the Treatment and Control groups in the pretest phase. This can be checked by running a two-sample t-test based on the data in range C4:C8 vs. C9:C13, as shown in Figure 3. Figure 3 – t-test on pretest data The p-value = .58 confirms that there is no significant difference between the Treatment and Control groups, although there is a larger medium-sized effect (d = .36) with a non-negligible difference between the group means (19 vs. 35). Repeated Measures ANOVAAnother approach is to perform a repeated-measures ANOVA with one between-subjects factor (treatment and control) and one within-subjects factor (pre- and post), as shown in Figure 4. Figure 4 – Repeated measures analysis Since there are only two between-subjects levels, we don’t need to worry about sphericity. For this analysis, p-value = .002875, which again shows that there is a significant difference between the Treatment and Control groups. Note that it is the p-value of the Interaction effect that is most relevant. Also, note that this value is identical to the p-value shown in cell K13 of Figure 2. This will be so for any similar analysis. The pretest-posttest control group design, also called the pretest-posttest randomized experimental design, is a type of experiment where participants get randomly assigned to either receive an intervention (the treatment group) or not (the control group). The outcome of interest is measured 2 times, once before the treatment group gets the intervention — the pretest — and once after it — the posttest. The objective is to measure the effect of the intervention which can be:
The pretest-posttest control group design has 3 major characteristics:
The pretest-posttest control group is the most commonly used design in randomized controlled trials. Advantages of the pretest-posttest control group designBy using a pretest, a control group, and random assignment, this design controls all internal threats to validity. Advantage of having a pretest measurementThis design is better than the posttest-only control group design because it adds a pretest. Adding a pretest:
Advantage of using random assignment and having a control groupRandom assignment and the control group will both limit the effects of:
Limitations of the pretest-posttest control group designParticipants included in any randomized study might not be typical people in the population i.e. they may not represent well the population of interest, this is because:
So the outcome of a randomized study may not generalize well to the population. More specifically, this design:
Example of a study that used the pretest-posttest control group designKoenig et al. used a pretest-posttest control group design to study the effect of a yoga program on the classroom behavior of autistic children. These children were randomly assigned to either receive the yoga program or their standard morning routine. The study concluded that yoga can significantly improve the classroom behavior of autistic children. But because the researchers used a convenience sample from a particular school and the classrooms that were allowed to participate were hand-picked by administrators, the study outcome may not generalize well to all children with autism. What is the pretest and posttest design in research?What Is Pretest-Posttest? A pretest is an assessment measure given to participants before they have undergone some type of treatment as part of a research study. A posttest is an assessment measure given to participants after they have received treatment as part of a research study.
What does a post test measure?Post-tests can measure resultant knowledge, competence, present or anticipated behavior, experienced or observed health outcome, or other topics. This is typically intended as a measure of immediately change or impact.
What does a pre/post experimental design requires the researcher to measure?The basic premise behind the pretest–posttest design involves obtaining a pretest measure of the outcome of interest prior to administering some treatment, followed by a posttest on the same measure after treatment occurs.
What is the purpose of the pretest in an experiment?Pretesting is the stage in survey research when survey questions and questionnaires are tested on members of target population/study population, to evaluate the reliability and validity of the survey instruments prior to their final distribution.
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