Item non-responses in mailed food frequency questionnaires in a Korean male cancer cohort study.

Item non-responses are frequently encountered in mailed food frequency questionnaires (FFQs) in epidemiological studies. The effects of item non-responses in a FFQ on presumed nutrient intake and subject classifications were examined in this study of a male cancer cohort in Korea. A semi-quantitative FFQ was developed and mailed to adult males aged between 40 and 59 in Seoul. Among the 14,533 cohort participants, 7,647 subjects who fully completed the FFQ initially and 216 subjects who completed the frequency missing items at resurvey were compared. When item non-responses were treated as not eaten, the average nutrient intakes of this group were significantly lower than both the average intake of subjects who fully completed at the initial survey and the average intake of those who completed at the resurvey. Increases in nutrient intakes during resurvey were substantively proportional to the number of items originally omitted. Cross classifications of item non-response subjects by nutrient intake after the initial survey and after resurvey showed misclassification towards lower quantile. Moreover, distribution of 'never or seldom' answer of resurvey group was similar to initial complete group. These results indicate that treating item non-responses as not eaten introduces bias when estimating nutrient intakes or when classifying subjects on the basis of nutrient intakes. More study is required to determine how best to treat non-response items in FFQ.