Two group, posttest only design

The logic behind the quasi-experimental design can also be applied to investigate factors that may have influenced an event that has already occurred.   Osman (1995) used this design in his investigation of the impact of the Rahad rural development project on fertility. He compared fertility rates in eleven villages that were resettled on irrigated farmland with fertility rates in six villages from the same region, but that did not receive irrigation for their lands. The two sets of villages were matched on a number of characteristics, based on data from a previous survey. Data on fertility were collected later from 853 women living in the villages with irrigation and from 356 women in the villages with dry lands. Fertility rates remained high among both groups of women, but recent rates had increased slightly more in the villages that benefited from irrigation. Another result was a greater decline in child deaths in villages with irrigated land.

One group, pretest and posttest design

This design is based on one group, a pretest measurement of a variable, the application of an experimental variable, and a posttest measure of the variable measured in the pretest. The example involving reduction of prejudice, used at the beginning of this chapter, was based on a one group design. Because no control group was used, we cannot say with certainty that any observed decline in levels of prejudice in the group studies was due to the program they experienced.

Sometimes, however, the one group design is the only one that can be used. Let's say we wanted to see if a workshop could make managers more sensitive to the needs of employees they supervise. We could develop two measures of an employee sensitivity scale, administer one at the beginning of the workshop, conduct the workshop, which would be our experimental variable, and then a week or so later, have the participants respond to the equivalent sensitivity scale.

With this design, at least we would have before and after data. The short time period involved would reduce, if not eliminate any threats associated with history, maturation or experimental mortality. Delaying the posttest for a about a week will help reduce reactivity and testing effect, although instrumentation and other threats would remain. These limitations would have to be acknowledged in a report describing this study.

For additional discussion of quasi-experimental designs, we suggest you visit:

Quasi-Experimental Design. This site presents information on quasi-experimental designs not covered in this chapter and has links to additional pages with information on quasi-experimental designs

True and Quasi-Experimental Designs. While this site focuses on designs used in evaluation, it has a lot of good, general information of both kinds of designs.

Overview: Experimental and Quasi-Experimental Research, an overview of basic concepts and an introduction to experimental research; described steps involved