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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.
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.