Contents

256 PART III: Experimental MethodsKey ConceptWhen two independent variables interact, we know that both variablestogether influence participants’ performance on the dependent variable, in thiscase, ratings of the interrogators’ effort to obtain a confession. Stated formally,an interaction effect occurs when the effect of one independent variable differsdepending on the level of a second independent variable. To understandthe interaction, examine the first row of Table 8.3. If only suspects who wereactually guilty had been tested in the experiment, we would have concludedthat the interrogators’ expectations had no effect on effort ratings because themeans for the guilty-expectation and innocent-expectation conditions arenearly identical. On the other hand, if only suspects who were actually innocenthad been tested (second row of Table 8.3), we would have decided thatinterrogator expectations had a large effect on interrogators’ efforts to obtain aconfession.An interaction effect is most easily seen when the means for the conditionsare graphed. Figure 8.1 plots the four means found in Table 8.3. These resultsindicate that ratings of the interrogators’ effort depend on whether the suspectis actually innocent or guilty and whether the interrogator expects thesuspect to be guilty or innocent—that is, both independent variables are necessaryto explain the effect. We describe the statistical analysis of interactioneffects in complex designs in a later section, “Analysis of Complex Designs.”For now, it is sufficient if you recognize that an interaction effect occurs when theeffect of one independent variable differs depending on the levels of a second independentvariable.When one independent variable interacts with a second independent variable,the second independent variable must interact with the first one (that is,the order of the independent variables doesn’t matter). For example, we describedthe interaction in Table 8.3 by stating that the effect of interrogators’expectations depends on the suspect’s status. The reverse is also true; the effectof suspect status depends on the interrogators’ expectations.FIGURE 8.1Graph illustrating the interaction effect between interrogator expectation and suspect status oneffort to obtain a confession. (Data provided by Dr. Saul Kassin.)Effort to Obtain Confession10987654321Actual GuiltActual InnocenceGuilty InnocentInterrogator Expectation