First‐order vs. second‐order structural validity of the Health Literacy Scale in patients with diabetes

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The term ‘health literacy’ was first mentioned in 1974, and research into this concept began in earnest during the 1990s 1. Health literacy was initially conceptualised within narrow perspectives focusing on the reading and numeracy skills needed to function adequately in a health setting. In contrast, the World Health Organization (2, p. 10) defined health literacy within a broader perspective as ‘the cognitive and social skills which determine the motivation and ability of individuals to gain access to understand, and use information in ways which promote and maintain good health’. In a similar vein, experts on health literacy reached a consensus in 2010/2011 that the concept of the health literacy is broader than just the functional health literacy of reading and calculating health information 3. Along with the broader concept, those experts recommended that the instrument should be improved by measuring more than functional health literacy, being based on theory, and possessing good psychometric properties.
Based on the World Health Organization definition, Nutbeam 4 proposed a model of health literacy that divided this into three types of literacy: functional health literacy (basic skills in reading and writing health information), communicative/interactive health literacy (cognitive and social skills in assessing and deriving health information), and critical health literacy (cognitive and social skills in analysing and using health information). Ishikawa et al. 5 used this model to develop the Health Literacy Scale (HLS) in Japanese patients with diabetes, which comprised functional, communicative, and critical subscales. The HLS is currently considered to be a comprehensive and useful instrument measuring health literacy for patients with diabetes 6.
However, a discrepancy in the structural validity of the HLS is emerging as a bothersome problem. Three subscales (factors) of the HLS were demonstrated in the original HLS study 6 and subsequent studies 7. Dwinger et al. 10 recently empirically demonstrated a two‐factor model, in which they combined the communication and critical subscales into one subscale due to the strong correlation between these subscales. This strong correlation between those two subscales is also a potential issue in the three‐factor model of the HLS.
A second‐order model is applicable when the first‐order factors are substantially correlated with each other 11. Based on this perspective, a second‐order model was proposed as the underlying structure of the HLS in the present study. To our knowledge, a second‐order model has never been investigated for the HLS. Therefore, the purpose of this study was to determine the structural validity of the HLS in Korean patients with diabetes based on the previously reported first‐order three‐factor and two‐factor models and a newly proposed second‐order model, to identify which model best represents the structure of the HLS.
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