In search of parsimony: reliability and validity of the Functional Performance Inventory-Short Form

Purpose: the 65-item Functional Performance Inventory (FPI), developed to quantify functional performance in patients with chronic obstructive pulmonary disease (COPD), has been shown to be reliable and valid. The purpose of this study was to create a shorter version of the FPI while preserving the integrity and psychometric properties of the original.

Patients and methods: secondary analyses were performed on qualitative and quantitative data used to develop and validate the FPI long form. Seventeen men and women with COPD participated in the qualitative work, while 154 took part in the mail survey; 54 completed 2-week reproducibility assessment, and 40 relatives contributed validation data. Following a systematic process of item reduction, performance properties of the 32-item short form (FPI-SF) were examined.

Results: the FPI-SF was internally consistent (total scale α = 0.93; subscales: 0.76-0.89) and reproducible (r = 0.88; subscales: 0.69-0.86). Validity was maintained, with significant (P < 0.001) correlations between the FPI-SF and the Functional Status Questionnaire (activities of daily living, r = 0.71; instrumental activities of daily living, r = 0.73), Duke Activity Status Index (r = 0.65), Bronchitis-Emphysema Symptom Checklist (r = -0.61), Basic Need Satisfaction Inventory (r = 0.61) and Cantril's Ladder of Life Satisfaction (r = 0.63), and Katz Adjustment Scale for Relatives (socially expected activities, r = 0.51; free-time activities, r = -0.49, P < 0.01). The FPI-SF differentiated patients with an FEV(l)% predicted greater than and less than 50% (t = 4.26, P < 0.001), and those with severe and moderate levels of perceived severity and activity limitation (t = 9.91, P < 0.001).

Conclusion: results suggest the FPI-SF is a viable alternative to the FPI for situations in which a shorter instrument is desired. Further assessment of the instrument's performance properties in new samples of patients with COPD is warranted.