Methodology makes a difference

In the United States, data from the National Health and Nutritional Examination Survey (NHANES) show sharp rises in the obesity rate among adults between 1976-1980 and 1988-1994, and again, between 1988-1994 and 1999-2002.²¹

In Canada, it is difficult to pinpoint when obesity rates increased, because of variations in the methods used to collect information on height and weight. Comparable obesity rates²² for adults aged 18 or older, based on directly measured height and weight, can be calculated for 1978/79, the 1986-1992 period, and 2004 (Chart 16) (see Data sources and analytical techniques). These data reveal little change in the obesity rate between 1978/79 and 1986-1992, but a substantial increase by 2004.

This rise is paralleled in self-reported data, although the percentages are consistently lower (Chart 16). Obesity rates based on self-reported data show a substantial increase between 1985 and 1994/95. Rates stabilized from 1994/95 to 2003.

Chart 16. Trends in obesity rates, household population aged 18 or older, Canada excluding territories, selected years, 1978/79 to 2004

Between 2003 and 2004, when the collection method changed from self-reported to directly measured data, obesity rates rose sharply. This is not surprising, as self-reports yield lower rates of overweight and obesity.^{9, 10, 11, 12} Women are inclined to underestimate their weight, while men tend to overestimate their height. Moreover, underreporting of weight increases with higher levels of BMI.²³

Another problem with overweight/obesity rates based on self-reports is variation in the mode of collection. Self-reported data from face-to-face interviews result in higher obesity rates than do data collected from telephone interviews.²⁴ In 1985 and 1990, all interviews were conducted by telephone; in 1994/95, almost all interviews took place in person; in 1996/97 and 1998/99, most were by telephone; in 2000/01, it was approximately half and half; and in 2003, about one-quarter were in person.