Item selection, reliability and validity of the Shortness of Breath with Daily Activities (SOBDA) questionnaire: a new outcome measure for evaluating dyspnea in chronic obstructive pulmonary disease

BACKGROUND: Chronic obstructive pulmonary disease (COPD) is characterized by irreversible, progressive obstruction of lung airflow. Dyspnea (shortness of breath [SOB]) is the COPD symptom which most negatively impacts patients’ daily activities. To assess how SOB affects daily activities, 37 items were drafted through focus group discussions and cognitive interviews with COPD patients to develop a patient-reported outcome instrument: the Shortness of Breath with Daily Activities questionnaire (SOBDA). Psychometric analysis was conducted to reduce the number of items and evaluate the measurement properties of the final SOBDA. METHODS: Prospective, observational study of 334 COPD patients, recruited from 24 pulmonology and internal medicine clinics in the United States. The 37-item SOBDA was administered to patients each evening for 28 days using an electronic diary. Patients answered every item and rated their level of SOB experienced that day during specific activities. Item selection was conducted by examining item characteristics, dimensionality, and Rasch model analysis results. The decision to delete an item was based on psychometric evidence, content validity, and expert clinical input. The final SOBDA instrument was evaluated for internal consistency, reproducibility, convergent validity, known-groups validity, and responsiveness. RESULTS: Twenty-four items from the 37-item pool were removed following the item selection process: nine items were removed due to high item-to-item correlations; five due to floor effects; three due to infrequent activity; one due to gender bias; two due to low factor loadings; three due to unordered response options; and one due to expert’s discretion. Internal consistency and reproducibility of the final SOBDA were demonstrated by Cronbach Alpha = 0.87, and intra-class correlation coefficient = 0.91. Convergent validity was demonstrated by high correlation with the CRQ-SAS (0.60) and SGRQ-C (0.61). Known groups validity was demonstrated by significant difference between ratings of the mMRC and clinical global rating of severity. Evaluation of the ability to detect change was not performed owing to too few responders at the end of the study. CONCLUSIONS: Through the empirical item reduction process, 13 items were selected from the 37-item pool generated during qualitative development. The final 13-item SOBDA is a reliable and valid instrument for use in clinical trials.