Test for item bias in a quality of life questionnaire.

Item bias (differential item functioning) analysis examines whether the construction of an index from two or more variables results in bias in relation to sex, age, or other criteria. Item bias may lead to erroneous conclusions because of distortion or dilution of the effects measured. In comparing groups, item bias analysis, tests whether the information about possible differences between groups, obtained by the variables constituting an index, are correctly passed on by the index score. We examined a quality of life questionnaire answered by 1189 breast cancer patients. We found age-bias or bias in the comparison of groups receiving different treatments in three out of nine indexes. Recommendations for the interpretation of these indexes are made. Item bias analysis is a useful method examining an issue not covered by traditional psychometric tests.