Beyond Rare-Symptoms Endorsement: a Clinical Comparison Simulation Study Using the Minnesota Multiphasic Personality Inventory-2 (MMPI-2) with the Inventory of Problems-29 (IOP-29)

To date, the MMPI-based, rare-symptom detection strategy is considered one of the most effective ones in symptom validity assessment. Because many of the items of the Inventory of Problems-29 (IOP-29) were designed specifically to provide incremental validity over the MMPI F scales, this study tested whether using the IOP-29 in combination with the MMPI-2 would provide higher classification accuracy compared to using either instrument alone. A total of 155 Italian adult individuals contributed to this study. About half (n = 93) were experimental malingerers (expMAL) instructed to simulate depression without being detected as feigners. The others were either (a) depressed patients in treatment (n = 36) or (b) individuals evaluated for possible malingering associated with work-related stress and considered to be genuinely affected by depression (n = 26). All were administered the Italian versions of both the MMPI-2 and the IOP-29. As expected, both instruments were highly effective in discriminating feigned from bona fide depression, with AUC values ranging from .77 to .90. More importantly, when entering the IOP-29 after each of the MMPI-2 scales under consideration (i.e., F, Fb, and Fp), the logistic regression models predicting group membership (0 = patient; 1 = expMAL) improved significantly. Likewise, each of the three MMPI-2 scales under consideration also significantly improved the prediction of group membership, when entered after the IOP-29. These findings thus indicate that using the MMPI-2 together with the IOP-29 could provide incremental validity over using either instrument alone, when testing depression-related complaints.