Objective: The study was conducted to assess the validity of a self-administered 150-item food frequency questionnaire (FFQ), used in a cohort study on diet and cancer (120,852 men and women, aged 55-69).
Design & subjects: The study was carried out in a subgroup of the cohort (59 men and 50 women) 2 years after the baseline FFQ was completed. A dietary record, kept over three 3-day periods, 4-5 months apart, served as reference method. To evaluate the representativeness of the study population for the entire cohort, a comparison was made with the baseline questionnaire of a random sample of the cohort.
Results: Pearson correlation coefficients between nutrient intakes assessed by the record and the FFQ that was completed afterwards ranged from 0.40 (95% CI: 0.22-0.54) for vitamin B1 to 0.86 (95% CI: 0.80-0.90) for alcohol intake, with correlations for most nutrients between 0.6 and 0.8. Adjustment for energy intake and sex did not materially affect these correlations, except the correlation for fat intake, which changed from 0.72 to 0.52. Correlation coefficients were only slightly modified when the results were extrapolated to the cohort at large. Correction of correlation coefficients for attenuation by day-to-day variance in the record data improved them by 0.07 on average.
Conclusions: It is concluded that the FFQ is able to rank subjects according to intake of food groups and nutrients. Despite a better performance of validation study participants, this conclusion also applies to the cohort at large.