Objective: To exemplify the construction of interval scales for specified categories of the International Classification of Functioning, Disability and Health (ICF) by integrating items from a variety of patient-oriented instruments.
Study design and setting: Psychometric study using data from a convenience sample of 122 patients with rheumatoid arthritis. Patients completed six different patient-oriented instruments. The contents of the instrument items were linked to the ICF. Rasch analyses for ordered-response options were used to examine whether the instrument items addressing the ICF category b130: Energy and drive functions constitute a psychometrically sound interval scale.
Results: Nineteen items were linked to b130: Energy and drive functions. Sixteen of the 19 items fit the Rasch model according to the chi-square (chi(2)) statistic (chi(2)(df=32)=38.25, P=0.21) and the Z-fit statistic (Z(Mean)=0.451, Z(SD)=1.085 and Z(Mean)=-0.223, Z(SD)=1.132 for items and persons, respectively). The Person Separation Index r(beta) was 0.93.
Conclusion: The ICF category interval scales to operationalize single ICF categories can be constructed. The original format of the items included in the interval scales remains unchanged. This study represents a step forward in the operationalization and future implementation of the ICF.