Exploring education-related differential item functioning in the EQ-HWB

The Experimental version (v1.1) EQ-HWB comprises 25 items that assess health- related, social care-related and caregiver-related quality of life across healthcare and social care systems. Individuals with lower educational levels may have difficulty understanding certain items, such as "control" and "cope with." Therefore, we examined the performance of these items and investigated education-related Differential Item Functioning (DIF) in a preliminary version of the EQ-HWB within the population.

Data were obtained from the second-round psychometric survey of the E-QALY project conducted in China using face-to-face paper-and-pencil administration and included 553 respondents with diverse educational backgrounds. Exploratory Factor Analyses (EFA) and Confirmatory Factor Analyses (CFA) were performed to examine the dimensional structure of the instrument. Item Response Theory (IRT) using the Graded Response Model was applied to evaluate item discrimination, difficulty, and item characteristic curves (ICCs). An Item Response Theory-Log-Likelihood Ratio Test (IRT-LRT) was used to assess DIF related to education.

The sample included 157 respondents with primary school education or below, 235 with secondary school education, and 161 with high school education or above (e.g. university). Factor analyses supported a three-factor structure with good model fit (CFI = 0.976, TLI = 0.970, RMSEA = 0.059). The items "Unable to cope with day-to-day life," "I felt that I had nothing to look forward to," and "I felt unsupported by other people" showed less clearly separated ICCs, suggesting that these items were difficult for this population. Of the 25 items, only two demonstrated education-related DIF (P < 0.001): "I felt I had no control over my day-to-day life" and "I could do the things I wanted to do."

Most EQ-HWB items functioned equivalently across educational groups, supporting the measurement invariance among respondents with different educational backgrounds. This study supports the validity of the EQ-HWB, especially in populations with low education levels.