Among the different types of experimental design are two general categories: true experimental designs and quasi- experimental designs. True experimental designs include more than one purposively created group, common measured outcomes, and random assignment. Quasi-experimental designs are commonly used when random assignment is not practical or possible. Frequently used quasi-experimental designs include: (1) the nonequivalent-group, posttest only design in which an outcome measure is administered to two groups or a program/treatment group and a comparison; (2) the nonequivalent-group, pretest-posttest design, in which differences between the two groups are initially assessed in a pretest; and (3) time-series designs in which several assessments or measurements are obtained from the treatment and control groups. True experimental designs yield the strongest comparisons, which include posttest only, control group designs and others designs, such as counterbalanced and matched subjects designs. The primary factor in the selection of an evaluation design is the purpose of the evaluation. It is essential to consider the usefulness of any evaluation information. The report recommends that evaluators use multiple evaluation methods as appropriate and be careful in collecting the right kinds of information when using experimental frameworks. The report also recommends considering alternative explanations for any observed differences in outcome measures. Program staff and participants can be helpful in identifying these alternative explanations and evaluating the plausibility of each. (Contains five references.) (SLD)