German version of the Illness Perception Questionnaire-Revised (IPQ-R): Psychometric evaluation in patients with chronic somatic illness

The objective of this study was the psychometric evaluation of the German version of the Illness Perception Questionnaire-Revised (IPQ-R). For this, factorial validity, construct validity, and concurrent criterion validity were determined and internal consistency and test-retest reliability were analyzed. The majority of the analyses were carried out in a cross-sectional study of N = 817 patients from six rehabilitation centers. The test for concurrent criterion validity was made on a partial sample of these patients contrasted with a sample of N = 58 acute patients. The test-retest reliability was determined using a sample of N = 45 patients from two rehabilitation centers. In general, the factorial structure of the original questionnaire was confirmed. The Cronbach's alpha coefficients are in the satisfactory to good range, with values between .70 and .87, but the scale "Treatment control" has an unacceptable value of alpha = .58. The pattern of the correlation matrix is plausible. The questionnaire discriminates between rehabilitation patients and acute patients and shows adequate test-retest reliability, with intraclass coefficients of between .55 and .87 and Pearson correlation coefficients between .56 and .87, respectively. Assessment of the cognitive illness representations of rehabilitation patients using the German IPQ-R is relatively reliable and valid. Some results indicate that the questionnaire could be optimized.