Pretest Posttest design refers to experimental studies where a researcher collects data on the subject before the experiment (called the pretest phase) and after the experiment (called the posttest phase). This kind of approach allows the researchers to measure the changes in the subject due to the experimental treatment and conclude whether the experimental treatment had any significant effect on the subjects or not.
To increase the accuracy of the experiment the researcher may also randomly divide the subjects into treatment and control groups to better understand the effects of the treatment.
The most well-known examples of pretest-posttest treatments are clinical trials. In a clinical trial, the researcher collects the health data of the individuals before a medicine is administered. The participants in the study are divided into treatment and control groups to account for the placebo effect. The participants are then given the medicine (or the placebo) and the health data are measured once again. By comparing the health data of the treatment and control groups before and after the medicine was given, the researcher can make conclusions about the efficacy of the medicine.
Posttest Only vs Pretest Posttest Designs
The main difference between posttest only and pretest-posttest design is that in a posttest only test we do not have a reference to which we can compare the data obtained after the experiment. In such a case it might become difficult to fully understand the effects of the treatments given to the participants.