Objectives: To develop a new rating scale for measuring the health impact of cervical dystonia (CD) that includes patients' perceptions and complements existing observer dependent clinician rating scales.
Methods: Scale development was in three stages. In Stage 1, a large pool of items was generated from patient interviews (n = 25), expert opinion, and literature review. In Stage 2, these items were administered by postal survey to people with CD. The resulting data were analyzed using Rasch item analysis to construct, from the item pool, a rating scale that satisfied criteria for rigorous measurement. In Stage 3, the measurement properties of this rating scale were examined in an independent sample of people with CD.
Results: In Stage 1, 150 items concerning the health impact of CD were generated. In Stage 2, 556 people completed questionnaires (87% response rate) and a 58-item rating scale measuring the health impact of CD in eight areas was constructed (CD Impact Profile, CDIP-58). In Stage 3, CDIP-58 data from 391 people (87% response rate) were received. Analyses supported the measurement of eight unidimensional constructs (infit mean square range 0.62 to 1.50), item calibration (33.37 to 67.56), and patient separation statistics (2.59 to 3.38). Items demonstrated stable calibrations in subgroups of people with CD supporting the stability of the CDIP-58.
Conclusions: The CDIP-58 is a reliable and valid patient-based rating scale measuring the health impact of CD in eight health dimensions.