Experimental research has been touted as one of the most rigorous research designs, due to a built-in safeguard for internal validity -- randomisation. A quasi-experimental design is very similar to an experimental research design, but lacks the key element of randomisation. Both designs feature an experimental group and a control group, but the manner of group selection differs. Therefore, the researcher ends up with non-equivalent groups. This design is referred to as a non-equivalent groups design (NEGD), the most common quasi-experimental design. Quasi-experimental designs offer some advantages and disadvantages.
Disadvantage One -- Potential for Non-Equivalent Groups
Using a sampling method other than random sampling increases the potential for constructing non-equivalent groups. Ideally, researchers endeavour to obtain experimental and control groups that are alike. This is most effectively achieved and most likely to occur through random selection. Quasi-experimental designs do not use random sampling in constructing experimental and control groups.
Disadvantage Two -- Potential for Low Internal Validity
Beginning research with non-equivalent groups presents a threat to internal validity. Internal validity refers to the degree to which a researcher can be sure that the treatment was responsible for the change in the experimental group. If the researcher does not start with equivalent groups, then the researcher cannot be sure that the treatment was the sole factor causing change. Other confounding factors may have contributed to the change. Therefore, not using random sampling methods to construct the experimental and control groups, increases the potential for low internal validity.
Advantage One -- Logistically Easy to Conduct
Quasi-experimental designs are often used in social research. These designs are often used in education to test the effectiveness of a program (treatment). In a typical quasi-experimental design, two classes may be selected, a pretest given to both, and then the treatment given to the experimental group. A post test is conducted to determine if there was a change in the groups. In education, these groups often come predetermined, such as in a school or class. Therefore, the researcher is not required to group individuals, as they come pre-grouped.
Advantage Two -- Control Group Comparisons Possible
Quasi-experimental research offers the benefit of comparison between groups. The experimental group is exposed to the treatment and the control group receives no treatment. The performance of the two groups can then be compared, after data is collected to determine if there was a difference in performance of the groups, after treatment. This is a major advantage because it helps the researcher to make inferences about the possible existence of a cause and effect relationship of the treatment.