Objective: We examined the presence and impact of differential item functioning (DIF) in a set of knee-specific functional status (FS) items administered using computerized adaptive testing (CAT) among English (United States) and Hebrew (Israel) speaking patients receiving therapy for knee impairments. DIF occurs in an item if probabilities of endorsing responses differ across groups after controlling for the FS measured.
Methods: We analyzed data from 28,320 patients (14,160 U.S., 14,160 Israel) who completed the knee-specific CAT. Items were assessed for DIF by gender, age, symptom acuity, surgical history, exercise history, and language spoken using a hybrid technique that combines multiple ordinal logistic regression and item response theory FS estimates.
Results: Several items had non-uniform DIF for covariates including language, but unadjusted and DIF-adjusted functional status estimates were in strong concordance [ICC(2,1) values >/=0.97], and differences between unadjusted and adjusted FS scores represented <0.4% of the unadjusted FS standard deviation.
Conclusions: Statistically significant DIF was identified in some items but represented negligible clinical impact. Results suggested no need to adjust items for DIF when assessing FS outcomes across groups of patients with knee impairments who answer the knee CAT items in English in the United States or Hebrew in Israel. These findings suggest negligible differences in cultural perceptions between English and Hebrew wording of these knee-specific CAT FS items.