Which of the following is not required in an experimental design?

In experimental design, a control group serves as a baseline to compare the effects of the independent variable on the treatment group, but it is not strictly required in every experimental design. The main goal of an experiment is to assess the impact of the treatment, and researchers can achieve this by using various designs, even without a distinct control group.

For instance, in some cases such as when ethical considerations prevent withholding treatment from participants or in smaller experiments, researchers may opt for a single-group, pre-test/post-test design where no explicit control group is utilized. This approach can still provide insights into the effects of the independent variable if measurements are made before and after treatment, allowing for the evaluation of changes within the same group.

In contrast, the other elements—blocking, random assignment, and independent variable—are fundamental to establishing a well-structured experimental approach. Blocking helps to account for variability among subjects by grouping similar units together, random assignment minimizes bias by evenly distributing characteristics across treatment groups, and the independent variable is essential as it represents the factor being tested. Thus, while a control group is highly beneficial for validity, it is not a mandatory component of every experimental design.