Cross-cultural applicability of the Inventory of Problems-29 (IOP-29): a pre-registered German replication of Akca, Tepedelen, et al. (2023)

Assessing the credibility of psychological problems in forensic evaluations is crucial. The Inventory of Problems-29 (IOP-29) is becoming a valuable symptom validity test (SVT) worldwide and has been validated in over 15 languages. This study evaluates the German IOP-29 and fills a gap by replicating a study of Akca, Tepedelen, et al. We analysed 384 IOP-29 protocols from 128 German-speaking adults (range = 18-87 years) under three conditions: honest, random and feigned responses (post-traumatic stress disorder, PTSD; depression; schizophrenia). Statistical analyses showed that the False Disorder Probability Score (FDS) effectively discriminated between honest and feigned responses (Hedges g = 3.90), with a sensitivity of .91 and a specificity of .95 at an FDS cut-off value of >=.50. A new index for detecting careless or random responses also showed promising results. This study confirms the utility of the IOP-29 in the German-speaking population and supports its cross-cultural applicability.