Validity of self-report screening for overweight and obesity. Evidence from the Canadian Community Health Survey.

OBJECTIVE Community health surveys often collect self-report data on body height and weight for the purposes of calculating the Body Mass Index (BMI) and identifying cases of overweight and obesity. The aim of the study was to test the validity of this method and to describe age and gender trends in self-report bias in height, weight, and BMI. METHODS This population survey included 4,615 adolescents and adults from across Canada who were interviewed and then measured in their homes. Overweight and obesity were identified using self-reports and cut points in BMI. RESULTS Self-reports correlated highly with body measurements but on average, self-reported height was 0.88 cm greater than measured height, self-reported weight was 2.33 kg less than measured weight, and BMI derived from self-reports was 1.16 lower than BMI derived from measurements. Consequently, self-reports yielded lower rates of overweight (31.87%) and obesity (15.32%) than measurements (33.67% and 22.92%, respectively). The magnitude and variability of self-report bias in BMI were related to female gender, older age, and the presence of overweight or obesity. DISCUSSION Comparison of self-reported and measured height and weight indicated that most survey respondents under-reported weight and over-reported height. Intentional or not, these biases were compounded in the BMI formula and affected the accuracy of self-reports as a tool for identifying weight problems. Self-reports may be easier to collect than body measurements but should not be used exclusively as an obesity surveillance tool.