Statistical analyses of Differential Item Functioning (DIF) can be used for rigorous
translation evaluations. DIF techniques test whether each item functions in the same
way, irrespective of the country, language, or culture of the respondents. For a given
level of health, the score on any item should be independent of nationality. This
requirement can be tested through contingency-table methods, which are efficient for
analyzing all types of items. We investigated DIF in the Danish translation of the
SF-36 Health Survey, using two general population samples (USA, n = 1,506; Denmark,
n = 3,950). DIF was identified for 12 out of 35 items. These results agreed with independent
ratings of translation quality, but the statistical techniques were more sensitive.
When included in scales, the items exhibiting DIF had only a little impact on conclusions
about cross-national differences in health in the general population. However, if
used as single items, the DIF items could seriously bias results from cross-national
comparisons. Also, the DIF items might have larger impact on cross-national comparison
of groups with poorer health status. We conclude that analysis of DIF is useful for
evaluating questionnaire translations.