Table 5 Differential item functioning according to age group, sex and disease type (n = 1286)

q1

F > M; p < 0.001

p = 0.492

SYM > P + S > PSY; p < 0.001

q2

F > M; p < 0.001

Y > A > O; p < 0.001

P + S > PSY; p < 0.001

q3

F > M; p < 0.001

Y > A > O; p < 0.001

P + S > PSY; p < 0.001

q4

F > M; p < 0.001

Y > O; p < 0.001

P + S > PSY > SYM; p < 0.001

q5

F > M; p < 0.001

Y > A > O; p < 0.001

P + S > PSY; p < 0.001

q6

F > M; p = 0.003

Y > A > O; p < 0.001

P + S > PSY > SYM; p = 0.044

q7

p = 0.055

A > Y > O; p = 0.004

P + S > PSY > SYM; p < 0.001

q8

F > M; p < 0.001

Y > A > O; p = 0.048

P + S > SYM > PSY; p < 0.001

q9

p = 0.300

Y > A > O; p < 0.001

P + S > SYM > PSY; p < 0.001

q10

F > M; p < 0.001

Y > A > O; p = 0.012

P + S > PSY; p < 0.001

DLQI

F > M; p < 0.001

Y > A > O; p = 0.004

P + S > PSY; p < 0.001