The Pain Disability Index: psychometric and validity data.

The Pain Disability Index (PDI) is a brief instrument that was developed to assess pain-related disability, providing information that complements assessment of physical impairment. This paper presents the results of two studies concerning the psychometric properties and the validity of the PDI. In study I, PDI scores of 108 patients appeared internally consistent (alpha = .86), although a factor analysis revealed two factors. The first factor (59.3% of variance) seemed to include more discretionary, less obligatory activities. The second factor (14.3% of variance) included activities more basic to daily living and survival. Study II found that the PDI scores of 37 former inpatients were significantly higher than 36 former outpatients who responded to a follow-up questionnaire. These findings support the validity of the PDI. Several methodologic issues are discussed, and suggestions are made for future uses of the instrument.