Estimates of obesity based on self-report versus direct measures
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by Margot Shields, Sarah Connor Gorber and Mark S. Tremblay
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.
body mass index, measurement error, misclassification, obesity, self-report, sensitivity and specificity, validity
Population health surveys often base estimates of the prevalence of obesity on calculations of body mass index (BMI), which is a measure of weight in relation to height. Since the mid-1990s, Statistics Canada's two major health surveys, the Canadian Community Health Survey (CCHS) and the National Population Health Survey (NPHS), have generally relied on respondents to report their weight and height and used these data to estimate BMI. [Full text]
Margot Shields (613-951-4177; Margot.Shields@statcan.gc.ca) and Sarah Connor Gorber (613-951-1193; Sarah.Connorgorber@statcan.gc.ca) are with the Health Information and Research Division, and Mark S. Tremblay (613-951-4385; Mark.Tremblay@statcan.gc.ca) is with the Physical Health Measures Division at Statistics Canada, Ottawa, Ontario, K1A 0T6.
- For fiscal and logistical reasons, self-reported height and weight data are collected in the large-scale health surveys conducted by Statistics Canada.
- It is important to document the extent to which the use of self-reported data biases estimates of overweight and obesity, and to identify factors associated with reporting error.
- The majority of studies have found that self-reports underestimate weight and overestimate height, resulting in lower estimates of the prevalence of obesity, compared with estimates based on measured data.
- In 2005, the estimate of the prevalence of obesity based measured data was 7 percentage points higher than the estimate based on self-reported data: 22.6% versus 15.2%.
- The degree of underestimation of weight in the 2005 CCHS was greater than that reported by other studies based on population health surveys conducted in the past in various countries.
- Over-reporting of height and under-reporting of weight increased with rising levels of BMI.
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