Fig. 2

Illustration of differential item functioning (DIF) by age. Expected mean item scores as a function of the latent trait score derived from item response theory (IRT) modelling, accounting for DIF. The left-hand panel displays the mean item score of item 8 as a function of the trait score for somatization, by age category. The right-hand panel displays the mean item score of item 26 as a function of the trait score for distress, by age category. The graphs were obtained from the program “lordif”