This self-contained and self-instructional unit is intended for use by evaluation and development personnel and by students in introductory research and evaluation courses. The unit contains a discussion of the regression employing graphic illustrations with actual data. The user is introduced to the regression effect in the single group pretest-posttests design, after which he responds to mastery test instructional exercises. The second part illustrates how the regression effect confounds the matched-pair type of design and this, too, is followed by mastery test instructional exercises. The user should be familiar with the basic statistical concepts of mean, standard deviation, correlation, and z-scores. (Author/SE)