A researcher has two options in assigning participants to control and experimental conditions: between subjects and within subjects.
For a between-subjects procedure, participants are randomly assigned to control and experimental groups. A between-subjects design ensures that participants are exposed to only one level of the independent variable—for example, a particular treatment—and not to any other types of treatment.
For a within-subjects procedure, all participants receive all conditions of the experiment. This procedure gives birth to what is called a repeated-measures design, also known as a within-subjects design.
Choosing a between- or within-subjects procedure depends largely on the research question under investigation, the nature of the independent and dependent variables, and the hypothesis being tested.
Data collected in psychological experiments are commonly analyzed by testing the statistical significance of the null hypothesis, such as there being no relationship between the independent and dependent variables.
The researcher aims to reject the null hypothesis in favor of the research hypothesis that predicts a specific relationship between the independent and dependent variables.
