O1 X O2
Before discussing the one-group prettest-posttest design, we should define some important terms:
Internal validity is the basic minimum without which any experiment is uninterpretable: "Did in fact the experimental treatments make a difference in this specific experimental instance?" External validity asks the question of generalizability: "To what populations, settings, treatment variables, and measurement variables can this effect be generalized?" Both types of criteria are obviously important, even though they are frequently at odds in that features increasing one may jeopardize the other. While internal validity is the sine qua non, and while the question of external validity, like the question of inductive inference, is never completely answerable, the selection of designs strong in both types of validity is obviously our ideal.
Now let us return to our discussion of the one-group pretest-posttest design. As we noted: while this design is still widely used in educational research, and while it is judged as enough better than Design 1 to be worth doing where nothing better can be done, it is introduced here as a "bad example" to illustrate several of the confounded extraneous variables that can jeopardize internal validity. These variables offer plausible hypotheses explaining an O1 - O2 difference, rival to the hypothesis that X caused the difference.
The one-group pretest-posttest design may be diagramed as follows:
O1 X O2
Under this design the researcher observes an event, behavior, response, etc. at one time and observes the same event, behavior, response, etc at a later point in time after a treatment, X. An example of Design 2 is the dropping of the A-bomb on Japan during World War II.
O1 X O2
X = Dropping of A-Bomb on Hiroshima and Nagasaki
O1 = Japanes at war with America O2 = Japanese surrender
Can you think of another explanation for the Japanese surrender? Let us examine the nine alternative explanations for our example.