To estimate reporting bias of WebCAAFE, a web-based questionnaire for the assessment of food intake (recall of frequency of intake of thirty-two food items the day before) and physical activity in schoolchildren.

Cross-sectional study. Self-reported food intake on WebCAAFE was compared with direct observation of school meals in five public schools. Additional data included school grade, sex, BMI, socio-economic status and access to Internet at home. Poisson regression was used to calculate the reporting bias (WebCAAFE v. direct observation) and the sample size necessary to detect a statistically significant difference between WebCAAFE reports and at least 75 % compliance with the recommendations for a healthy diet.

Intentional sample of five elementary public schools in Florianopolis, Brazil.

Schoolchildren (n 629) from 2nd to 5th grades.

Moderate bias magnitude was found for most food groups of interest. Frequency of consumption was not related to the bias. Sample sizes necessary to detect the compliance with dietary recommendations varied between four and seventy-four individuals for the different groups investigated.

After adjusting for moderate bias, WebCAAFE may be used as a food questionnaire for evaluation of schoolchildren’s food compliance on a group level, even with a relatively small sample size.