Psychometric validation of the French version of two scales measuring general (HLS₁₉-Q12) and navigational (HLS₁₉-NAV) health literacy using the Rasch model

Background: To examine the psychometrics properties of the French version of two scales measuring general (HLS₁₉-Q12) and navigational (HLS₁₉-NAV) health literacy (HL) using validation methods based on modern psychometric test theories: a Rasch model analysis.

Methods: The data on representative samples of the French adult population came from the Health Literacy Survey (N = 2 003), conducted in France in two waves (2020 and 2021), and from the third wave of SLAVACO study (N = 2 022), conducted in December 2021. A Rasch analysis was performed using a partial credit model with marginal maximum likelihood estimation adapted to polytomous data. Category probability curves were used to examine whether participants found it consistently difficult to distinguish between response options. Differential item functioning (DIF) was also examined for each item in the two scales as a function of age, gender, and economic status. A Person Separation Index (PSI) of 0.7 has conventionally been considered to be the minimum acceptable PSI level. Chi-square fit statistics, expressed as infit and outfit mean square statistics, were calculated for each item in the two scales. An acceptable fit corresponded to average values between 0.7 and 1.3. The dimensionality of the scales was assessed using Principal Components Analysis (PCA) on the residual correlation matrix to identify whether items are locally independent. Additionally, Smith's method was employed to detect potential multidimensionality.

Results: The PSI values for both scales were greater than 0.90. No disordered categories were observed. No evidence of significant DIF was found when associated with gender and economic status. However, two items for both scales had a DIF which was dependent on age but did not have a significant impact on scale scores between younger and older people. Minor local dependence was noted on the HLS₁₉-NAV scale. However, a PCA and Smith's method demonstrated the unidimensionality of the French version of the scales measuring general and navigational HL.

Conclusions: Our results, based on a rigorous statistical analysis, verified the psychometric parameters of the French version of the HLS₁₉-Q12 and HLS₁₉-NAV scales. Despite strong correlation between both scales, each measured a different latent trait. Moreover, despite the fact that presence of DIF was weak, attention must nonetheless be paid when comparing scale response scores between young and older respondents.