Although the randomized trial is considered the optimal design for testing cause-and-effect hypotheses, the necessary restrictions of a randomized trial are not always possible within the clinical environment.1 Depending on the nature of the treatment under study and the population of interest, use of randomization and control groups may not be feasible. The specification of inclusion and exclusion criteria will often reduce generalizability and limit the range of patients that can be included.
Quasi-experimental designs utilize similar structures to experimental designs, but will lack either random assignment or comparison groups, or both. They often involve nonequivalent groups that may differ from each other in many ways in addition to differences between treatment conditions.2 Therefore, the degree of control is reduced. Many studies incorporate quasi-experimental elements because of the limitations of clinical conditions. These designs present reasonable alternatives to the randomized trial, as long as the researcher carefully documents subject characteristics, controls the research protocol, and uses blinding as much as possible. The conclusions drawn from these studies must take into account the potential biases of the sample, but may provide important information, nonetheless.3
One-Group Pretest-Posttest Design
The one-group pretest-posttest design is a quasi-experimental design that involves one set of repeated measurements taken before and after treatment on one group of subjects (see Figure 11.1). The effect of treatment is determined by measuring the difference between pretest and posttest scores. In this design, the independent variable is time, with two levels (pretest and posttest). Treatment is not an independent variable because all subjects receive the intervention.
A one-group pretest-posttest design. Because only one group is tested and all subjects receive the intervention, time becomes the independent variable.
Example of a One-Group Pretest-Posttest Design
A study was designed to examine the effect of four-direction shoulder stretching exercises for patients with idiopathic adhesive capsulitis.4 All subjects received the same exercise protocol. Researchers studied the effects of treatment on pain, range of motion, function and quality of life measures. Comparisons were made between pretest scores and final scores at follow-up, with a mean duration of 22 months.
In this study, the researchers saw significant improvements in outcome variables, and concluded that the treatment was successful. We must hold conclusions drawn from this design as suspect, however. The design is weak because it has no comparison group, making it especially vulnerable to threats to internal validity. Although the researcher can demonstrate change in the dependent variable by comparing pretest and posttest scores, there is always the possibility that some events other than the experimental treatment occurred within the time frame of the study that caused the observed change. Therefore, this design is particularly threatened by history and ...