A two-group random assignment pretest–posttest design is an experimental design that compares measures of a dependent variable (outcome) before and after the introduction of an independent variable (some experimental manipulation or intervention) between two groups with randomly selected participants. This design is often used in studies where researchers are interested in establishing a causal connection between the manipulation and the outcome. This entry first introduces the concept of experimental research and underscores the importance of internal validity, then explains this particular design and how it contributes to internal validity of experiments, explores the types of conclusions researchers using this method are able to draw, and finally offers some problems and solutions related to this method.