Objectives: Routine data collection is now considered mandatory. Therefore, staff rated clinical scales that consist of multiple items should have the minimum number of items necessary for rigorous measurement. This study explores the possibility of developing a short form Barthel index, suitable for use in clinical trials, epidemiological studies, and audit, that satisfies criteria for rigorous measurement and is psychometrically equivalent to the 10 item instrument.
Methods: Data were analysed from 844 consecutive admissions to a neurological rehabilitation unit in London. Random half samples were generated. Short forms were developed in one sample (n=419), by selecting items with the best measurement properties, and tested in the other (n=418). For each of the 10 items of the BI, item total correlations and effect sizes were computed and rank ordered. The best items were defined as those with the lowest cross product of these rank orderings. The acceptability, reliability, validity, and responsiveness of three short form BIs (five, four, and three item) were determined and compared with the 10 item BI. Agreement between scores generated by short forms and 10 item BI was determined using intraclass correlation coefficients and the method of Bland and Altman.
Results: The five best items in this sample were transfers, bathing, toilet use, stairs, and mobility. Of the three short forms examined, the five item BI had the best measurement properties and was psychometrically equivalent to the 10 item BI. Agreement between scores generated by the two measures for individual patients was excellent (ICC=0.90) but not identical (limits of agreement=1.84+/-3.84).
Conclusions: The five item short form BI may be a suitable outcome measure for group comparison studies in comparable samples. Further evaluations are needed. Results demonstrate a fundamental difference between assessment and measurement and the importance of incorporating psychometric methods in the development and evaluation of health measures.