Validation of subscales of the Severe Asthma Questionnaire (SAQ) using exploratory factor analysis (EFA)
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RESEARCH
Validation of subscales of the Severe Asthma Questionnaire (SAQ) using exploratory factor analysis (EFA) Joseph W. Lanario1, Michael E. Hyland1,2* , Andrew Menzies‑Gow3, Adel H. Mansur4, James W. Dodd5, Stephen J. Fowler6, Rupert C. Jones1 and Matthew Masoli7
Abstract Background: The Severe Asthma Questionnaire (SAQ) is a health related quality of life (HRQoL) questionnaire vali‑ dated for use in severe asthma. It is scored using the mean value of 16 items (SAQ score) in addition to a single item global rating of HRQoL (SAQ-global). The aim was to validate clinically relevant subscales using exploratory factor analysis (EFA). Methods: The SAQ was completed, along with measures of asthma control and EQ5D-5L by patients attending six UK severe asthma centres. Clinical data were included in the analysis. EFA using principal axis factoring and oblimin rotation was used to achieve simple structure of data. Results: 460 patients with severe asthma participated, 65% women, mean age 51 (16–83) years. A three factor solution achieved best fit and showed that the SAQ items formed three distinct but inter-correlated groups of items where items were grouped in a way that was consistent with item content. The three subscales were differentially associated with clinically relevant variables (lung function and mood). Males and females interpreted the question of night disturbance in different ways. Conclusions: This paper provides a template for best practice in the use of EFA when validating HRQoL subscales. The SAQ can be scored as three subscales with content reflecting three different constructs people with severe asthma use when making judgements about their lives. The subscale ‘My Life’ assesses the impact of severe asthma on different life activities, ‘My Mind’ assesses the perceived emotional impact and ‘My Body’ the impact of extra-pul‑ monary symptoms and side effects. Keywords: Asthma, Quality of life, Measurement, Exploratory factor analysis Background Validated health-related quality of life (HRQoL) questionnaires are used in clinical practice and research to evaluate the impact of disease and/or treatment responses. They consist typically of several items the responses to which are aggregated to form an overall HRQoL score. Subscales can be formed from groups of *Correspondence: [email protected] 1 University of Plymouth, Plymouth, UK Full list of author information is available at the end of the article
items as subscales provide information that can be useful in clinical trials and clinical practice. Guidelines for validating questionnaires recommend a two stage process where content validity is followed by construct validity [1], but these recommendations were published after the publication of four asthma specific HRQoL questionnaires. Validation of the asthma quality of life questionnaire’s (AQLQ) subscales is based only on content validity [2, 3] as the subscales are formed by grouping items on the basis of an examination of content alone. However, in three other asthma speci
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