Key Concepts About Population Counts
Calculating population counts for a given condition from NHANES follows these steps:
- Calculate the percentage who have the outcome or characteristic by age, sex, or race/ethnicity subgroups,
in which you are interested. You will output these results to a SAS or
Stata file.
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 Note: Age standardization of the
prevalence estimates is NOT performed because the population counts should be
based on the crude (unadjusted) prevalence in the population.
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- Use the relevant population totals from the Current Population Surveys
(CPS) to determine population estimates in NHANES. Since NHANES is a nationally
representative survey of the non-institutionalized U.S. population,
population estimates are based on the CPS totals for this aspect of the
U.S. population. Use CPS totals for the midpoint of each survey cycle.
CPS-based population tables for NHANES by race/ethnicity, gender and age are
located at:
http://www.cdc.gov/nchs/nhanes/response_rates_CPS.htm.
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Note: Population totals generated
in NHANES can only be representative of the number of individuals with the health condition in the non-institutionalized U.S. population.
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- If you
wish to report multiple age, gender or race/ethnic subgroups, you can combine
these population totals. It also is possible to combine NHANES survey cycles.
For example, to combine two survey cycles (e.g., 2001-2002 and 2003-2004), you
must use the midpoint of each cycle, and combine them as follows: ½ (NHANES
2001-2002 population totals) + ½ (NHANES 2003-2004 population totals) in order
to get a population total for 2001-2004. Similarly, you would do this for each
of the age-, sex-, or race/ethnicity groups you wanted to combine to get a
population total for that group.
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Note: The
only exception would be when combining NHANES 1999-2000 with 2001-2002 data. As
stated in the weighting module, these survey years used a different reference
population for sampling, so population totals for 1999-2002 are provided by
NCHS.
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Once CPS
totals are combined, results should be output to a
file.
- Multiply the prevalence of the health condition of interest by
the corresponding CPS-based population total to obtain an estimate of the
number of non-institutionalized U.S. individuals with the condition. To
calculate age-, sex-, or race/ethnicity- specific population estimates,
multiply the prevalence of the health condition in each sub-domain by the
CPS population total for the respective sub-domain.
Since the non-institutionalized CPS population totals are
used to calculate the final sampling weights for the NHANES survey, you may
wonder why you cannot just sum the final sampling weights for all sample persons
with the health condition of interest, in order to arrive at population
estimates for the health condition. For example, the total population estimate
for a given health condition from the interviewed sample should equal the sum of
the final interview weights for that health condition within the demographic
domains among all interviewed persons. However, if there are a significant
number of exclusions or missing data for a health condition, summing the weights
will not produce an accurate population estimate. Therefore, using this
method is NOT RECOMMENDED. The differences in population estimates by the
calculated method versus the summed weight method are illustrated in the
table below.
Comparison of Population Estimates using Calculated and
Summed Methods
| Sample Domain |
% U.S. Population |
Correct Estimate |
Incorrect Estimate |
| Total |
29.2% |
57,859,000 |
55,362,000 |
| Male |
27.3% |
25,844,000 |
24,855,000 |
| Female |
30.9% |
32,039,000 |
30,506,000 |
| Non-Hispanic Blacks |
37.0% |
8,103,000 |
7,277,000 |
| Mexican American |
17.1% |
2,409,000 |
2,182,000 |
 DO
NOT use the summed weight method to determine population estimates
for a given health condition because the potential for exclusions or
missing data for that health condition may lead to population
underestimates. |
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