Examining the Factor Structure of Objective Health Literacy and Numeracy Scales

Background: Scales for measuring health literacy and numeracy have been broadly classified into performance-based (objective) and self-reported (subjective) scales. Both types of scales have been widely used in research and practice; however, they are not always consistent and may assess different latent constructs. Furthermore, an increasing number of objective measures have been developed and it is unclear how many latent factors should be assumed. Therefore, we aimed to examine the psychometric properties and factor structure of items assessing objective health literacy across multiple scales and to clarify which aspects of objective health literacy would be correlated with subjective measures, as well as health behaviors and lifestyles. Methods: Five objective scales (72 items in total) were administered to Japanese-speaking adults (N = 16,097; 7722 women; mean age = 54.89). The analyzed scales included items assessing the numeracy, comprehension, and application of health information, some of which were contextualized for specific diseases such as diabetes and cancer. Participants’ responses were submitted to exploratory factor analysis, and individual factor scores were calculated to test correlations with subjective health literacy, health behavior, and lifestyle. Results: Exploratory factor analysis identified three factors, which were interpreted as numeracy, comprehension, and synthesis, respectively. All numeracy items loaded onto the same factor, even when contextualized for different diseases. The comprehension factor consisted of items about medical word comprehension, and the synthesis factor was characterized by items assessing the ability to read and understand health-related information and make judgments on it using their own knowledge. The identified factors showed high inter-factor correlations (rs = 0.54–0.67) and small-to-moderate correlations with subjective health literacy (rs = 0.15–0.44). Additionally, each factor indicated small positive correlations with healthy diet and nutrition and less substance use (rs = 0.19–0.26). Conclusions: Our findings suggest that scales of objective health literacy have at least three latent constructs (numeracy, comprehension, and synthesis) and that disease specificity is not psychometrically prominent. Each factor has some overlap with subjective health literacy, but overall, subjective and objective health literacy should be interpreted as independent constructs given the small-to-modest correlations.