Discussion
In this study, we first simplified the CRHLS (2018 edition) based on IRT and DIF methods. Finally, the simplified version of the remaining 27 items for older adults was adapted with a good reliability and even slightly improved in construct validation. The original scale, widely used in the periodic national survey to monitor the level and dynamic changes of health literacy in Chinese adults aged 15–69 years old, was developed on the manual of “Basic Knowledge and Skills of People’s Health Literacy” published by the National Health and Family Planning Commission (Recently, IRT was more and more applied to assess and revise the scales in public health or clinical studies (We simplified the Chinese instrument for measuring health literacy by removing some redundant or DIF items to finally form the simplified version with 27 items focused on older adults in China. However, there were some limitations. Firstly, IRT model needed to meet basic assumptions of unidimensionality, monotonicity and local independence. In this study, the data was verified to satisfy these basic assumptions only except for the eigenvalue examination in the dimension of health-related skills (the ratio of the first to second eigenvalue was 2.52, which slightly less than 3 of threshold value). Secondly, because the original version was initially developed by an expert panel from the Ministry of Health and now was widely used for community-based adults aged 15–69 years old in China, we did not further assess the content validity in this study. Moreover, the items kept in short version were selected based on IRT and DIF. The additional items recommended by researchers might be subjective, so did not be considered in this study. Thirdly, we used the data of the representative sample of older adults from a cross-sectional study conducted in Beijing. There might be differences in characteristics among older adults living in different regions, which might have an impact on the reliability and validity of the scale when it was extended used in general Chinese older adults. So, this simplified version needs to the further assess in the general populations.