A method for shortening instruments using the Rasch model. Validation on a hand functional measure.

Background: Many measurement instruments, particularly measures of hand functional ability, frequently comprise a large number of items. Reduced versions of these instruments can facilitate their use. This work proposes a new method for shortening an instrument.

Methods: The method proposed was based on a scale of item difficulty calculated using the Rasch model. It was applied on a hand functional measure comprising 67 tests. The sample included 194 patients with hand lesions. The shortened instrument obtained was compared with those provided by classic methods used in the literature, with item random choice, and with shortened versions proposed by four independent experts, two rehabilitation physicians and two occupational therapists, who are clinicians familiar with the tool. All the statistical analyses were carried out on a random sub-group of two-thirds of the sample. A cross validation was then carried out on the remaining third.

Results: The reduction obtained had score non significantly different from that of the original instrument. In addition, the intra-class correlation coefficient and the Cronbach alpha coefficient were high. Among the different degrees of reduction investigated, the 12-item version seemed to be appropriate. Our method appeared to provide better results in terms of discriminant validity and internal validity than the choices of the four experts. The reductions produced were also better than those obtained by classic methods based on principal component analysis and multiple linear regression, as well as those obtained by random choices of items.

Conclusion: The method presented is pertinent and useful. The reduction obtained appeared to be better than the choices of experts and the reductions provided by classic methods. The method could be used in other fields.