One of the requirements of the Rasch model is that the scale should be coordinated between different subgroups (e.g. age and gender).
34 Because all the subjects included in this study were Chinese, there was no difference in the item function in terms of ethnicity. To determine whether there was any difference in the response among other characteristics of the participants, DIF analysis was conducted through three dimensions as follows: (1) online versus telephone administration, (2) gender (male versus female), and (3) age (>49 years versus ≤49 years, because the mean age of the participants was 49 years old). Because the DIF analysis was performed on 150 items simultaneously to test the hypothesis of no difference in any of the items across different groups, we applied the Bonferroni correction to the tests for 150 items of the scale, so the
P value for statistical significance was adjusted from 0.05 to 0.0003.
13,35,36 Along any of the three dimensions examined, no items reached statistical significance, thus no evidence for model differences was revealed. Then, we went through those with
P values < 0.05 and |DIF contrast| >1.0 logit (see
Supplementary Materials S5) along the 3 aforementioned dimensions, and made reasoned judgment for any evident DIF.
37 We noticed that only when comparing female with male subjects, 9 items (item 20, 37, 38, 41, 60, 81, 86, 105, 140, and 9/150 = 0.06) demonstrated |DIF contrast| >1.0 logit (female versus male, ranged = −1.26 −1.11, mean = −0.40, SD = 1.11) with
P < 0.05. For a 150-item scale, the potential impact of these items on the measurement could be negligible.
35,38 The items were inspected one by one, and there was no obvious bias against male or female subjects visually. Therefore, the Chinese ULV-VFQ-150 is largely free of DIF.