Comparison of scaling methods for activities of daily living in older people

Abstract

Objective: To compare the psychometric properties of scales top measure activities of daily living, constructed with different scaling methods, and to check whether the most complex scales have higher discriminatory capacity. Method: Sample of elderly people from the Spanish Survey on Disability, Personal Autonomy and Dependency We used 14 items that measured activities of daily living. Five scaling methods were applied: Sum and Rasch (both for dichotomous and polytomous items) and Guttman (dichotomous). We evaluated the discriminatory capacity (relative precision [RP]) and area under the curve (AUC). Results: All methods showed high Pearson correlations among them (0.765-0.993). They had similar discriminatory power when comparing extreme categories of individuals with no disability with severely limited (RP: 0.93-1.00). The polytomous Sum procedure showed the highest AUC (0.934; 95% confidence interval [95%CI]: 0.928-0.939) and Guttman the lowest (0.853; 95%CI: 0.845-0.861). Conclusions: Polytomous items have greater reliability than the dichotomous ones. Simplest methods (Sum) and most complex (Rasch) are equally valid. Guttman method presented worse discriminatory capacity.