Quasi-experimental Design

The prefix quasi means "similar to." The quasi-experimental design is similar to true experimental research in that variables are manipulated, but participants are not assigned to conditions at random. For these reasons, it has a higher internal validity than correlational studies but a lower internal validity than true experiments. Non-equivalent group designs, pre-test/post-test designs, and interrupted time series designs are all important types of quasi-experimental design. A non-equivalent group design is a between-subjects design in which participants are not assigned to conditions at random. The dependent variable is measured twice in a pre-test and post-test design, once before and once after the intervention is implemented. The interrupted time-series design is a variation on the pre-test and post-test design. A time series is a collection of measurements taken at regular intervals over time. This design is similar to a pre-test and post-test design in that dependent variables are measured both before and after treatment, but it differs because it includes multiple pre-test and post-test measurements. A non-equivalent group design or a pretest-posttest design that combines both is another type of quasi-experimental design that is better in comparison to both. A treatment group is given a pre-test, treatment, and post-test. In addition, a control group is given a pre-test, no treatment, and post-test. The question then becomes not whether participants who receive treatment improve, but whether they improve more than participants who do not.