Chapter 17: Quasi-Experimental Designs

Introduction

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. Depending on the nature of the treatment under study and the population of interest, use of randomization and control groups may not be possible.

Quasi-experimental designs utilize similar structures to experimental designs, but lack either random assignment, comparison groups, or both. Even with these limitations, these designs represent an important contribution to clinical research because they accommodate for the limitations of natural settings, where scheduling treatment conditions and random assignment are often difficult, impractical, or unethical. They are often used in pragmatic studies because of the logistic limitations that occur in practice. The purpose of this chapter is to describe time series designs and nonequivalent group designs, the two basic structures of quasi-experimental research.

Validity Concerns

Because quasi-experimental designs lack at least one of the requirements for controlled trials, they cannot rule out threats to internal validity with the same confidence as experimental studies. Nonequivalent groups may differ from each other in many ways in addition to differences between treatment conditions. Therefore, the degree of control is reduced.

Quasi-experimental 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.

Time Series Designs

Many research questions focus on the variation of responses over time. In such a design, time becomes an independent variable with several levels and researchers will look for differences across time intervals. Such designs can be configured in several ways, with varying degrees of control.

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 Fig. 17-1). 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.