The purpose of an experimental design is to provide a structure for evaluating the cause-and-effect relationship between a set of independent and dependent variables. Within the design, the researcher manipulates the levels of the independent variable and incorporates elements of control, so that the evidence supporting a causal relationship can be interpreted with confidence.
Although experimental designs can take on a wide variety of configurations, the important principles can be illustrated using a few basic structures. The purpose of this chapter is to present these basic designs and to illustrate the types of research situations for which they are most appropriate. For each design, we discuss strengths and weaknesses in terms of experimental control and internal and external validity. In addition, we include a short statement suggesting general statistical procedures for analysis. These suggestions do not represent all statistical options for a particular design, but they do represent the more commonly used techniques. This information demonstrates the intrinsic relationship between analysis and design.
The term clinical trial is often used to describe experimental studies that examine the effect of interventions on patient or community populations. Clinical trials are frequently designed on a large scale, involving subjects from a range of geographic areas or from several treatment centers. Clinical trials can be classified as either therapeutic or preventive. Therapeutic trials examine the effect of a treatment or intervention on a particular disease. For example, 25 years of clinical trials, begun in the 1970s, have shown that radical mastectomy is not necessary for reducing the risk of recurrence or spread of breast cancer, and that limited resection can be equally effective in terms of recurrence and mortality.1 A preventive trial evaluates whether a procedure or agent reduces the risk of developing a disease. One of the most famous preventive trials was the field study of poliomyelitis vaccine in 1954, which covered 11 states.2 The incidence of poliomyelitis in the vaccinated group was over 50 percent less than among those children who received the placebo, establishing strong evidence of the vaccine's effectiveness. In a more contemporary example, scientists continue to design trials in an effort to develop a vaccine to prevent HIV infection.3
In the investigation of new therapies, including drugs, surgical procedures and electromechanical devices, distinct sequences of clinical trials are typically carried out. The phases of trials are intended to provide different types of information about the treatment in relation to dosage, safety and efficacy, with increasingly greater rigor in demonstrating the intervention's effectiveness and safety (see Box 10.1).
Experimental designs can be described according to several types of design characteristics. A basic distinction among them is the degree of experimental control.4,5 In a true experimental design, subjects are randomly assigned to at least two comparison groups. An ...