Estimates of obesity based on self-report versus direct measures

Based on a representative sample of the Canadian population, this article quantifies the bias resulting from the use of self-reported rather than directly measured height, weight and body mass index (BMI).

The analysis is based on 4,567 respondents to the 2005 Canadian Community Health Survey (CCHS) who, during a face-to-face interview, provided self-reported values for height and weight and were then measured by trained interviewers.

On average, males over-reported their height by 1 cm, and females, by 0.5 cm. Females under-reported their weight by an average of 2.5 kg; males, by 1.8 kg. Reporting bias in weight was strongly associated with measured BMI category. Under-reporting of weight was high among people who were overweight, and particularly high among those who were obese, compared with people of normal weight. When based on measured rather than on self-reported values, the prevalence of obesity was 9 percentage points higher among males and 6 points higher among females.