Figure 1

Differential item functioning (DIF) between people with PD and PAD. Examples of two NHPD items (panel A, item 4/"unbearable pain"; panel B, item 6/"feeling on edge") displaying cross-diagnostic DIF. The item characteristic curves (ICCs; grey curves) represent the expected probabilities of item endorsement (y-axis) at various levels of the measured construct (x-axis). Superimposed plots represent the observed responses by people with PD and PAD, as divided into six class intervals according to their levels of illness-related distress. Observed differences indicate that items do not work the same way in the two diagnostic groups. For comparison, panel C illustrates an item without DIF (item 14/"feel as if losing control").