The Posttest-Only Design with Random Assignment

Look carefully at figure 4.1d. It is the second half of the Solomon four-group design and is called the Campbell and Stanley posttest-only design. This design has a lot going for it. It retains the random assignment of participants in the classical design and in the Solomon four-group design, but it eliminates pretesting—and the possibility of a confound

FIGURE 4.1d.

The Campbell and Stanley posttest-only design.

from pretest sensitization. When participants are assigned randomly to experimental conditions (control or treatment group), a significant difference on O₁ and O₂ in the posttest- only design means that we can have a lot of confidence that the intervention, X, caused that difference (Cook and Campbell 1979).

Another advantage is the huge saving in time and money. There are no pretests in this design and there are only two posttests instead of the four in the Solomon four-group design.

Here’s an example of this elegant design. McDonald and Bridge (1991) asked 160 female nurses to read an information packet about a surgery patient whom they would be attending within the next 8 hours. The nurses were assigned randomly to one of eight experimental conditions: (1) The patient was named Mary B. or Robert B. This produced two patient-gender conditions. (2) Half the nurses read only a synopsis of the condition of Mary B. or Robert B., and half read the same synopsis as the fourth one in a series of seven. This produced two memory-load conditions. (3) Finally, half the nurses read that the temperature of Mary B. or Robert B. had just spiked unexpectedly to 102°, and half did not. This produced two patient stability conditions. The three binary conditions combined to form eight experimental conditions in a factorial design (more on factorial designs at the end of this chapter).

Next, McDonald and Bridge asked nurses to estimate, to the nearest minute, how much time they would plan for each of several important nursing actions. Irrespective of the memory load, nurses planned significantly more time for giving the patient analgesics, for helping the patient to walk around, and for giving the patient emotional support when the patient was a man (box 4.3).