Reproducibility of a food frequency questionnaire used in the New York University Women's Health Study: effect of self-selection by study subjects.

OBJECTIVE

The aim of the study was to investigate the reproducibility of a food frequency questionnaire (FFQ) used in a cohort of 14,290 women enrolled in the NYU Women's Health Study.

DESIGN

A subset of 474 cohort subjects who completed the dietary questionnaire at baseline (FFQ-1) were approached again on the occasion of a second visit to the mammography study centre and asked to complete the questionnaire a second time (FFQ-2) two to four years after FFQ-1. Two to three months later the questionnaire was mailed to the subjects, and they were asked to complete it a third time (FFQ-3).

SETTING

A breast cancer screening clinic.

SUBJECTS

Of the 474 subjects selected, 100% completed the second questionnaire while only 56% completed and mailed back FFQ-3. This made it possible to compare the long-term reproducibility of dietary intake measurements and baseline dietary habits between the two groups of subjects 'respondents', who agreed to complete the questionnaire a third time, and 'non-respondents', who did not.

RESULTS

Among respondents (56% of study subjects), energy-adjusted correlation coefficients for short-term reproducibility between FFQ-2 and FFQ-3 ranged from 0.50-0.64 for nutrients, and from 0.44-0.67 for foods. The long-term reproducibility was lower, ranging from 0.36-0.53 for nutrients, and from 0.31-0.48 for specific food groups. Among those who did not respond to FFQ-3, crude correlations for long-term reproducibility, unadjusted for energy intake, were generally lower than among respondents. Nevertheless, after adjustment for energy intake, correlations for long-term reproducibility (FFQ-2 to FFQ-1) were of similar magnitude in both groups. In addition, 'non-respondents' reported lower intake of fruit and vegetables and higher intake of meat.

CONCLUSIONS

The results of this study suggest that subjects who volunteer to participate in substudies on the validity or reproducibility of dietary questionnaire measurements may tend to provide more accurate responses to the questionnaire. The phenomenon seems related more to accuracy of reporting of absolute intake levels than of the relative composition of diet, self-selection may be associated with differences in dietary habits.