Developing the HLS(19)-YP12 for measuring health literacy in young people: a latent trait analysis using Rasch modelling and confirmatory factor analysis

BACKGROUND: Accurate and precise measures of health literacy (HL) is supportive for health policy making, tailoring health service design, and ensuring equitable access to health services. According to research, valid and reliable unidimensional HL measurement instruments explicitly targeted at young people (YP) are scarce. Thus, this study aims at assessing the psychometric properties of existing unidimensional instruments and developing an HL instrument suitable for YP aged 16–25 years. METHODS: Applying the HLS(19)-Q47 in computer-assisted telephone interviews, we collected data in a representative sample comprising 890 YP aged 16–25 years in Norway. Applying the partial credit parameterization of the unidimensional Rasch model for polytomous data (PCM) and confirmatory factor analysis (CFA) with categorical variables, we evaluated the psychometric properties of the short versions of the HLS(19)-Q47; HLS(19)-Q12, HLS(19)-SF12, and HLS(19)-Q12-NO. A new 12-item short version for measuring HL in YP, HLS(19)-YP12, is suggested. RESULTS: The HLS(19)-Q12 did not display sufficient fit to the PCM, and the HLS(19)-SF12 was not sufficiently unidimensional. Relative to the PCM, some items in the HLS(19)-Q12, the HLS(19)-SF12, and the HLS(19)-Q12-NO discriminated poorly between participants at high and at low locations on the underlying latent trait. We observed disordered response categories for some items in the HLS(19)-Q12 and the HLS(19)-SF12. A few items in the HLS(19)-Q12, the HLS(19)-SF12, and the HLS(19)-Q12-NO displayed either uniform or non-uniform differential item functioning. Applying one-factorial CFA, none of the aforementioned short versions achieved exact fit in terms of non-significant model chi-square statistic, or approximate fit in terms of SRMR ≤ .080 and all entries ≤ .10 that were observed in the respective residual matrix. The newly suggested parsimonious 12-item scale, HLS(19)-YP12, displayed sufficiently fit to the PCM and achieved approximate fit using one-factorial CFA. CONCLUSIONS: Compared to other parsimonious 12-item short versions of HLS(19)-Q47, the HLS(19)-YP12 has superior psychometric properties and unconditionally proved its unidimensionality. The HLS(19)-YP12 offers an efficient and much-needed screening tool for use among YP, which is likely a useful application in processes towards the development and evaluation of health policy and public health work, as well as for use in clinical settings. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1186/s12913-022-08831-4.