A psychometric assessment of the St. George's respiratory questionnaire in patients with COPD using rasch model analysis

Background: The St. George's Respiratory Questionnaire (SGRQ) was a widely used tool to assess disease impact on patients with obstructive airways disease. Although traditional methods have generally supported construct validity and internal consistency reliability of SGRQ, such methods cannot facilitate the evaluation of whether items are equivalent to different individuals. The purpose of this study is to rigorously examine the psychometric properties of the SGRQ in patients with chronic obstructive pulmonary disease (COPD) using Rasch model analysis. Methods: A methodological research was conducted on SGRQ in a sample of 240 male patients with COPD recruited from the outpatient services in Central Taiwan. The psychometric properties of the SGRQ were examined using Rasch model analysis with a mixed rating scale and partial credit mode by Winsteps software. The level of matching between the item's difficulty and person's ability was analyzed by item-person targeting as well as ceiling and floor effects. Item-person maps were also examined for checking the location of the item's difficulty and person's measures along the same scale. Finally, the differential item functioning (DIF) was examined to measure group equivalence associated with age and disease's severity. Results: Each of the three domains (Symptom, Activity, Impact) of the SGRQ was found to be unidimensionality. The person separation index ranged from 1.21 (Symptom domain) to 2.50 (Activity domain). There was a good targeting for the SGRQ domains, except the Impact domain (1.36). The percentage of ceiling and floor effects were below 10%, except the ceiling effect in the Impact domain (26.25%). From item-person maps, gaps of location of item corresponded to patient's ability were identified. The results have also showed that many items in SGRQ revealed age or severity related DIF. Conclusions: Except the Symptom domain of SGRQ, the others have a reliabile internal consistency and a good hierarchical structure. The results of Rasch model analysis can highlight aspects for scale improvement, such as gap, duplicate items or scale responses. There was some age or severity related DIF indicating somewhat unstable across different characteristics of group.