Self-administered semiquantitative food frequency questionnaires: patterns, predictors, and interpretation of omitted items.

BACKGROUND: Food items on a self-administered food frequency questionnaire (FFQ) may be left blank because the food was not consumed, because of difficulties remembering the frequency or amount of intake, or due to an oversight.
METHODS: We explored the predictors and frequency of consumption of omitted food items on an FFQ used in the Nurses' Health Study II. Of 87,676 women who returned a mailed 147-item FFQ in 1999, 34% completed the entire questionnaire, whereas 66% left at least 1 food item blank. Ten or more food items were omitted by 5% of participants. Foods were more likely omitted by women who were older or more physically active. We resurveyed 2876 participants who had left between 1 and 70 food items blank and asked them to fill in the blanks. Overall, 2485 participants provided complete responses.
RESULTS: In the resurvey, 64% of the formerly omitted foods were marked as consumed never or less than once per month, 20% as 1 to 3 times per month, 8% as once per week, and 9% as more than once per week. Commonly consumed foods and beverages were less likely omitted because they were not consumed than rarely consumed foods. The best estimate for the true intake value of an omitted food was 0.82 times the average population intake.
CONCLUSIONS: While calculating nutrient intake, the assumption that items missing represent zero intake is reasonable. However, food items consumed more often in the population at large are less likely than rarely consumed food items to be left blank because they were not consumed.