Two-Group Experimental Designs

The simplest of all experimental designs is the two-group posttest-only randomized experiment. In design notation, it has two lines – one for each group – with an R at the beginning of each line to indicate that the groups were randomly assigned. One group gets the treatment or program (the X) and the other group is the comparison group and doesn’t get the program (note that this you could alternatively have the comparison group receive the standard or typical treatment, in which case this study would be a relative comparison).

Notice that a pretest is not required for this design. Usually we include a pretest in order to determine whether groups are comparable prior to the program, but because we are using random assignment we can assume that the two groups are probabilistically equivalent to begin with and the pretest is not required (although you’ll see with covariance designs that a pretest may still be desirable in this context).

In this design, we are most interested in determining whether the two groups are different after the program. Typically we measure the groups on one or more measures (the Os in notation) and we compare them by testing for the differences between the means using a t-test or one way Analysis of Variance (ANOVA).

The posttest-only randomized experiment is strong against the single-group threats to internal validity because it’s not a single group design! (Tricky, huh?) It’s strong against the all of the multiple-group threats except for selection-mortality. For instance, it’s strong against selection-testing and selection-instrumentation because it doesn’t use repeated measurement. The selection-mortality threat is especially salient if there are differential rates of dropouts in the two groups. This could result if the treatment or program is a noxious or negative one (e.g., a painful medical procedure like chemotherapy) or if the control group condition is painful or intolerable. This design is susceptible to all of the social interaction threats to internal validity. Because the design requires random assignment, in some institutional settings (e.g., schools) it is more likely to utilize persons who would be aware of each other and of the conditions they’ve been assigned to.

Threat typeLevel of threat to Two-Group Experimental Designs
History✔ Weak
Maturation✔ Weak
Testing✔ Weak
Instrumentation✔ Weak
Mortality✔ Weak
Regression to the mean✔ Weak
Selection✔ Weak
Selection-History✔ Weak
Selection-Maturation✔ Weak
Selection-Testing✔ Weak
Selection-Instrumentation✔ Weak
Selection-Mortality❌ Severe
Selection-Regression✔ Weak
Diffusion or Imitation❌ Severe
Compensatory Equalization❌ Severe
Compensatory Rivalry❌ Severe
Resentful Demoralization❌ Severe

The posttest-only randomized experimental design is, despite its simple structure, one of the best research designs for assessing cause-effect relationships. It is easy to execute and, because it uses only a posttest, is relatively inexpensive. But there are many variations on this simple experimental design. You can begin to explore these by looking at how we classify the various experimental designs.