A quasi experimental design looks like experimental design, however this design lacks the randomized group assignment.
Manipulation Something is “done to” the subjects, such as prescribed a new medication or a different method of teaching.
Control/comparison group Members of this group are not assigned randomly. They could be similar students in another school, or patients on another ward in the hospital
Examples of quasi experimental design
Nonequivalent control group design In this research design, the control or comparative group has limited resemblance to the intervention group. This group could be comprised of similar subjects in another locale
Time series design In this design, the same group serves as intervention and control. Data are analyzed at different time periods. For example, Weight loss was measured three months before the intervention and three months after the intervention.
Any statistical tests that compare groups, such as independent T tests, chi square tests for categorical variables (examples gender, race, age group, geography). In the case of a crossover design, the statistical tests will also need to measure within group differences. ANOVA (analysis of variance) is one such test.
Strengths and limitations
Quasi experimental designs provide a pragmatic view of research. It may be easier to find a non-equivalent control group than to find a substantial number of subjects who agree not to have the intervention. However, from a rigorous, scientific perspective, a quasi-experimental design is not as good as an experimental design because the comparison group is not equivalent.