BMI is a variable with four columns and two implied decimal places.
For sample adults, BMI reports the Body Mass Index, a measure of body fat based on height and weight that applies to adult men and women.
Calculation of BMI
The Codebooks for 1997 forward provide information about the calculation of BMI by the National Center for Health Statistics (NCHS) and about the interpretation of the results.
For 2004-2005, this information was phrased as follows:
The wording in the Codebooks for 1997-2003 was slightly different, but the substantive meaning was the same as in the text quoted above.
The conversion factors for 2006 forward were slightly different, with 1 kilogram defined as 2.20462 pounds and 1 meter defined as 39.37008 inches.
As the basis for BMI, the survey collected information about the individual's height and weight. For height, interviewers asked, "How tall are you without shoes?"; for weight, interviewers asked, "How much do you weigh without shoes?" These results are reported in HEIGHT and WEIGHT, respectively.
In the public use files, for 1997 forward, the heights and weights of people with unusually high or low values were suppressed, to protect respondents' confidentiality. As mentioned in the quote above, to calculate BMI, the NCHS used the full range of weight and height values for respondents, not the restricted range of values included in the public use data files.
Apart from the slight difference in conversion factors for 2006 forward relative to those used in1997-2005, this variable appears to be comparable over time. The exact formula for calculating BMI in 1991, 1993, and 1995 was not specified in the NHIS public use documentation or annual reports for those years, however.
According to the Field Representative's Manuals for 1991, 1993, and 1995, if the respondent reported that she was pregnant, or if this was known from earlier responses, the interviewer was to "determine the person's weight before she became pregnant and record it," and to put the word "pregnant" and the person's present weight in a footnote. If the BMI figures for prior to 1997 had been calculated using pre-pregnancy weight for pregnant women, this would compromise comparability with BMI figures for 1997 forward. However, the BMI variable in the public use files for 1991, 1993, and 1995 is labeled "Body Mass Index- Current Weight," so presumably present weight (rather than pre-pregnancy weight) was used in the calculations.
The BMI figures in the public use files for 1991, 1993, and 1995 were reported in three digits with one implied decimal place, while the BMI figures for 1997 forward were reported in four digits with two implied decimal places. Using supplemental programming, the IPUMS NHIS project converted all the BMI data into a consistent format of four columns with two implied decimal places.
BMI is only available in the public use NHIS data files for the years specified above, but researchers may wish to use such a statistic for earlier years as well. With this in mind, IHIS calculated a second measure of BMI, BMICALC (Body Mass Index, calculated from height and weight), for 1976 forward.
Results for BMICALC will differ from those for BMI.
For BMICALC, IHIS had to use the smaller range of weight and height values included on the public use data files. BMICALC does not report a Body Mass Index figure for persons whose weight or height was topcoded, bottomcoded, or suppressed as an outlying value. In addition, BMICALC was calculated using slightly different formula: [Weight in pounds/(Height in inches, squared)] multiplied by 703 and rounded to one digit past the implied decimal point.
Researchers interested in data for 1991, 1993, 1995, and/or 1997 forward only should use BMI rather than BMICALC. Researchers interested in data from other years must use BMICALC or must calculate the Body Mass Index themselves from the WEIGHT and HEIGHT variables. Researchers interested in data from both before and after 1997 may wish to use BMICALC to maximize comparability over time.
- 1991: Sample persons age 18+.
- 1993: Half of sample persons age 18+ in quarters 3 and 4 (excluded from AIDS supplement).
- 1995: Half of sample persons age 18+ (excluded from AIDS supplement)
- 1997-2017: Sample adults age 18+.
- 1991-1993, 1995, 1997-2017