In a properly designed experiment, what is the purpose of random assignment?

Random assignment is a fundamental technique used in experimental design to ensure that participants are evenly and randomly distributed across different treatment groups. The primary purpose of random assignment is to eliminate bias and control for confounding variables. By randomly assigning subjects to various groups, researchers can minimize the impact of external factors that could influence the results, thereby increasing the internal validity of the experiment.

This process helps to ensure that the groups being compared are statistically similar before any treatment or manipulation is applied. As a result, any observed differences in outcomes can be more confidently attributed to the treatment itself rather than to pre-existing differences between the groups.

This aspect of random assignment is crucial for establishing cause-and-effect relationships, as it strengthens the conclusion that changes in the independent variable directly lead to changes in the dependent variable, rather than being influenced by other, uncontrolled factors.