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For 1976, the recognized range was 50 to 300 pounds; for 1977-1981, 50 to 400 pounds; for 1982-1995, 50 to 500 pounds; for 1996, 97 to 290 pounds. In 1997 forward, the recognized range for weight differed between men and women. For 1997-2005, the range for men was 126 to 285 pounds; for women, 99 to 259 pounds. For 2006 forward, the range for men was 126 to 299 pounds; for women, 100 to 274 pounds. According to the 2006 Survey Description Document, the upper threshold for weight was raised for both men and women and the lower threshold was raised for women beginning in 2006 "due to the increasing numbers of overweight and obese adults."

For years prior to 1997, persons who weighed more than the top-coded value or less than the bottom-coded value were included in the highest and lowest categories, respectively. Thus, for example, in 1996, category 290 includes people who weighed 290 pounds or more. By contrast, for 1997 forward, persons who whose weight was exceptionally low or high were grouped together in a separate "Exceptionally low or high weight" category (code 996). As the Survey Description Document for 1997 explained, "Beginning in 1997, when a very large or very small value was reported for either height or weight, the data in both variables were changed to '96' or '996' ('Not available') on the public use data files. This was done to protect the confidentiality of NHIS respondents who might be identifiable by their unusual physical characteristics."

For example, average weight calculated using 1976 data would necessarily be less than average weight calculated using 1977 data, because the topcodes for the two years differ (300 pounds versus 400 pounds). For comparisons across periods, the best approach would be to limit the sample to persons whose weight falls within the smallest recognized range for the period under study and exclude persons assigned the top or bottom codes.