A two-group pretest–posttest design is an experimental design, which compares the change that occurs within two different groups on some dependent variable (the outcome) by measuring that variable at two time periods, before and after introducing/changing an independent variable (the experimental manipulation or intervention). This entry describes the purpose and setup of this specific type of experimental design, compares it to similar designs, explores its relative advantages as well as potential pitfalls, and finally suggests some improvements for increased internal validity.