Table 4.

Differential item functioning in the original 10-item DLQI version

Differential item functioning	Results (Original 10-item DLQI)
Age	Item 2: more agreement among people < 35 (p = .028)
	Item 4: more agreement among people < 35 (p = .015)
Sex	No DIF
Location	No DIF
Education	Item 4: more agreement for higher education (p = .038)
Severity of illness	Item 4: more agreement for patients with more severe condition (p = .027)
	Item 8: more agreement for patients with more severe condition (p = .041)

The criterion for differential item function was a Mantel–Haenszel statistic with p < 0.01 after Bonferroni correction [21]. Using an uncorrected p < 0.05 is not common, but minimizes the risk of underestimating item bias. After Bonferroni correction, no differential item functioning was observed