Correction of the self-reported BMI in a teenage population

OBJECTIVE: To investigate the issue of systematic bias in self-reported weight and height, and produce a simple procedure which can be used to correct reporting bias. DESIGN: Cross-sectional, with self-reported questionnaires. SUBJECTS: A sub-sample (n = 143) of secondary school students in Siena, Italy, taken from the Food Behaviour Survey (sample size, n = 779). RESULTS: In the teenage sub-sample, both males and females under-reported their weight and over-reported their height, such that underestimation of the overweight prevalence was in the order of about 8% for both genders. For both weight and height, the correlations between self-reported and measured values were over 0.90. Conversion factors were derived to correct the reported body mass index (BMI) distribution by adjusting the percentages of erroneously classified subjects in the four BMI categories. CONCLUSION: High correlation coefficients (r greater than or equal to 0.75), showing a systematic tendency for erroneous self-reporting of a 'slim-body shape', justify the use of conversion factors (measured/self-reported) to correct BMI distributions calculated from self-reported values.