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254 PART III: Experimental MethodsTABLE 8.1A MAIN EFFECT OF INTERROGATOR EXPECTATION ON THENUMBER OF GUILT-PRESUMPTIVE QUESTIONSInterrogator ExpectationSuspect Status Guilty InnocentActual guilt 3.54 2.54Actual innocence 3.70 2.66Means for interrogatorexpectation 3.62 2.60Hypothetical cell means based on Kassin et al. (2003).in the innocent-expectation condition. Thus, they predicted a main effect of theinterrogator-expectation independent variable.The data for this dependent variable, number of guilt-presumptive questionsselected, are presented in Table 8.1. The overall mean number of guiltpresumptivequestions for participants in the guilty-expectation condition (3.62)is obtained by averaging the means of the actual-guilt and actual-innocence conditionsfor interrogators in the guilty-expectation condition: (3.54 3.70)/2 3.62. Similarly, the overall mean for the innocent-expectation condition is computedto be 2.60: (2.54 2.66)/2 2.60. 1 The means for a main effect representthe overall performance at each level of a particular independent variable collapsedacross (averaged over) the levels of the other independent variable. In this case wecollapsed (averaged) over the suspect status variable to obtain the means forthe main effect of the interrogator expectation variable. The main effect of theinterrogator-expectation variable is the difference between the means for thetwo levels of the variable (3.62 2.60 1.02). In the Kassin et al. experiment,the main effect of the interrogator-expectation variable indicates that the overallnumber of guilt-presumptive questions selected was greater when interrogatorsexpected a guilty suspect (3.62) than when they expected an innocentsuspect (2.60). Inferential statistics tests confirmed that the main effect of interrogatorexpectation was statistically significant. This supported the researchers’hypothesis based on behavioral confirmation theory.Let’s now turn to a dependent variable for which there was a statisticallysignificant main effect of the suspect-status independent variable. The researchersalso coded the tape-recorded interviews to analyze the techniques used bythe interrogators to obtain a confession. Student interrogators were given brief,written instructions regarding the powerful techniques police use to breakdown a suspect’s resistance. Researchers counted the number of interrogatorstatements that reflected these persuasive techniques, such as building rapport,assertions of the suspect’s guilt or disbelief in the suspect’s statements, appeals1The simple averaging of the values within each row and column to obtain the means for themain effects is possible only when there are equal numbers of participants contributing to eachmean in the table. For procedures to calculate weighted means when the cells of the table involvedifferent sample sizes, see Keppel (1991).