Test–retest reliability and external validity of the previous day food questionnaire for 7–10-year-old school children

The aim of this study was to assess the reliability and external validity of the Previous Day Food Questionnaire (PDFQ) designed to obtain a report of the foods eaten on the previous day by schoolchildren. Participants were 7–10-year-old school children of the first four grades of a public school in Southern Brazil (N = 227). Test–retest reliability was evaluated by kappa coefficient for two administrations of the PDFQ on the same day to the same children. External validity of the PDFQ was evaluated via sensitivity, specificity, and positive and negative predictive values (PPV and NPV, respectively) using trained observers of the food eaten on the previous day as gold standard. The association between responses from observed food intake with those from reported food intake on PDFQ was evaluated by multivariate logistic regression, controlled for school grade, gender, time of the eating, and the variation between first and second PDFQ applications. For the reliability study, the analyses stratified by school eating occasions (3 a day) indicated that agreement level was moderate or better for all food categories. PDFQ's sensitivity ranged from 57.1% (vegetables) to 93.3% (rice), whereas its specificity ranged from 77.8% (bread/pasta) to 98% (meats). Both, PPV and NPV were reasonably high. PDFQ was highly associated with observed food intake, with effect magnitude several times larger than any other factor analyzed for all foods. PDFQ also showed good test–retest reliability, suggesting that it may generate reliable and valid data for assessing food intake at the group (school) level.