randomized posttest control group

Definition
[...] is one of the simplest yet strongest (sampling) designs in the educational researcher's arsenal, at least from the perspective of control over biasing influences.

Notes

The basic building block of experimental designs is the randomized posttest control group design shown below (where R refers to random assignment, T refers to a treatment or experimental intervention, C refers to a control or comparison, and Post refers to a posttest):
R T Post
R C Post (Suter 2012, pg. 5)

The essential ingredients of a randomized posttest control group design are the use of a treatment group that receives a specific intervention, a posttest to measure (assess) the influence of the treatment effect (if any), a control group to rule out threatening sources of influence, and random assignment of subjects to the control and treatment groups. (pg. 5)

Example
Here is another example of the randomized posttest control group design. Let's assume that a researcher wanted to test the effectiveness of an educational intervention designed to reduce the number of high school students who smoke. Assume that 1,000 ninth graders were selected to participate and 500 were randomly assigned to receive the treatment, consisting of information about the hazards of smoking, including guest speakers with terminal lung cancer. The remaining 500 students were assigned to the control group and were not targeted in any special way. The posttest measure was collected near the end of high school and was simply a count of the number of students in each group who smoked regularly.
Here are the dramatic results: treatment group = 6% smokers; control group = 20% smokers. These hypothetical results favor the intervention, and there are no obvious alternative explanations for the findings. Even if there were powerful extraneous influences—such as the continual release of new studies showing the health consequences of smoking, the implementation of substantially lower car insurance rates for nonsmokers, or the smokingrelated death of a popular entertainer—the findings would still provide a strong causal link between the educational intervention and the reduced smoking rate. That is because the extraneous influences affected the groups equally. The presence of a control group allows the researcher to “subtract out” extraneous influence to arrive at a purer measure of the treatment's effect. If there were no control group, then we would not know whether the low smoking rate in the treatment group was due to the intervention itself, to the smoking-related death of the teen idol, or to a combination of both or any one of the hundreds of other plausible explanations related to extraneous influences.