Pretest-Posttest Change

Pretest-posttest change can be studied at the population level and at the individual level. Within-group change is the change of a population parameter, for example, the mean from pretest to posttest. Between-groups change is the difference of within-group change between (e.g., E- and C-) groups. In contrast, within-person change is the change of a single person. In general, change at the population level does not apply to single persons. Within-group change is assessed by testing the null hypothesis of equal pretest and posttest population means. Between-groups change can be tested with different methods. The test based on posttest-pretest difference scores is described. Single-person change is derived from the participant’s observed test scores or his (her) item responses. Change methods based on item responses depend on the scale of the items. Methods for continuous and dichotomous item responses are described. The participant’s observed pretest and posttest scores are used to test the null hypothesis of no true score change. The same null hypothesis can be tested with the participant’s item responses if these responses are at a continuous scale. The method uses the participant’s continuous item responses instead of his (her) test scores. An individual change measure is defined for dichotomous item responses. This measure is applied to test the null hypothesis of no change of a single person. It is recommended to study pretest-posttest change at both the population and the individual level because these levels yield different information.