Interventional research is, quite simply, research where it is possible to intervene in treatment.

IMPORTANT NOTE: The whole REASON for experimental and quasi-experimental design is to control for extraneous factors.

ANOTHER IMPORTANT NOTE: The KEY difference between experimental designs and quasi-experimental designs is randomization. This will be discussed more fully under experimental designs.

EXPERIMENTAL DESIGNS1

The prototypical "true" experiment, where the experimenter has virtually total control over the research situation and experimental subjects, randomly assigning some patients to one or more treatment possibility(s) and some to one or more control group(s), is the highest form of interventional research. The randomized control trial (RCT) and N of 1 RCT are members of this class.

Other names you may encounter for true experimental designs are:

• The Pretest-Posttest Control Group Design
• The Solomon Four-Group Design
• The Posttest-Only Control Group Design
• Factorial designs

QUASI-EXPERIMENTAL DESIGNS

But there are other forms of interventional research where the experimenter has less than the ideal of total control over the situation, as in the practice-based research situation. This can occur due to the infeasibility of randomization, ethical problems (e.g., withholding a preferred treatment from a control group), etc. Quasi-experimental research designs have been developed for these less-than-perfect circumstances. You can, of course, use randomization with these designs, but they can also be used in situations where randomization is not possible or not advisable in order to help control for extraneous factors. But please keep in mind that these designs do not control for extraneous factors as well as randomized clinical trials and that some quasi-experimental designs are stronger than others.

(Do you get the idea that an understanding of the problem of extraneous factors may be of some importance?)

Some examples of quasi-experimental designs are:

• Time Series Design
• Control Group Time Series Design
• Equivalent Time Samples Design
• Equivalent Materials Design
• Non-equivalent Control Group Design
• Counterbalanced Designs
• Separate Sample Pretest-Posttest Design
• Separate Sample Pretest-Posttest Control Group Design
• Multiple Time Series Design
• Recurrent Institutional Cycle Design

PRE-EXPERIMENTAL DESIGNS

The primary pre-experimental designs are the:

• One-shot Case Study
• Static Group Comparison
• One Group Pretest-Posttest

These designs are not very strong, do not offer any design-specific control for extraneous factors, and are generally best reserved for exploratory, pilot types of studies, if used at all.

Campbell, D.T. and Stanley, J.C. (1966). Experimental and Quasi-experimental Designs for Research. Chicago: Rand McNally.