The National Health and Nutritional Examination Survey (NHANES)

The CDC has a number of great national surveys. Last week I talked about the BRFSS, a large scale annual survey.  But perhaps the most widely known survey is known as NHANES.  This survey is conducted in waves over several years. It consists of two teams that travel across the country, settling in one place for about six weeks, and extensively testing carefully selected survey respondents. Again, the survey methodology is very critical because it informs us of how to use the survey and what are its limitations.

Survey respondents are selected in advance of the arrival of the survey team. They are sent letters and invited to participate. Those who agree to participate arrive at the survey site where over a two day process they are extensively surveyed and give it a number of lab tests including the drawing of blood and the taking of urine samples. Respondents are not randomly selected. The CDC uses a cluster sampling methodology and oversampled blacks and Hispanics. The survey includes both children and adults.

Analyzing the data requires special software that has the appropriate survey commands that can accommodate the cluster sampling design. Standard statistical packages such as SAS and STATA have these commands. Simple analytical packages may not. A major limitation of NHANES is that it is not geographically representative of the United States. The sample selected to be demographically representative, but because the two teams could only visit a total of 16 places a year, is impossible to achieve a good geographic spread. This means if you want to know differences between parts of the country it just doesn’t work. Therefore, the survey is best used for national data or maybe for very large states or groups of states. It’s also not very good for looking at changes over time because one doesn’t know if the changes because of the geographic irregularities of the survey. About a year ago, it was heralded the news that the US obesity rate had appeared to have leveled off. This was based on NHANES data and so those of us in the know were rather skeptical whether the leveling off represented a real change or was just a function of a different geography participating in a newer wave of subjects; we just didn’t know.

Again however, these limitations are what they are. NHANES has provided vital information on a number of health issues including, for example, the buildup of fire retardant chemicals in adults and children. It was this survey that helped identify the problem with self-report of height and weight. The NHANES researchers asked subjects to report their height and weight but also had trained professionals measure these things. Thus it was found that people lie! Or at least misrepresent their height and weight.  NHANES has been used as the source for thousands of studies and is an invaluable part of the arsenal of what people use to identify environmental and health problems in the United States.

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