Divide your subjects in half. one group receives one treatment of the independent variable and the other group receives a differ
ent treatment of the independent variable. subjects were all told they were going to see a video of a therapist's session after which they would rate the quality of the session. the groups differed in that the subjects in one group were told that prior evaluations indicated that the therapist was effective whereas subjects in the other group were told that the evaluations indicated the therapist was not effective. these different subjects were used for the two levels of the independent variable: subjects were in either the "effective therapist" or the "ineffective therapist" condition
