Key Concepts About NHANES II Survey Design

NHANES II data are NOT obtained using a simple random sample.  Rather, a complex, multistage, probability sampling design is used to select participants representative of the civilian, non-institutionalized US population. The sample does not include persons residing in nursing homes, members of the armed forces, institutionalized persons, or U.S. nationals living abroad.

NHANES II Sampling Procedure

The NHANES II sampling procedure consists of 4 stages, shown and described below.

Four Stages of NHANES Sampling Procedure

• Stage 1: Primary sampling units (PSUs) were selected.  These were mostly single counties or, in a few cases, groups of contiguous counties with selected probability proportional to a measure of size (PPS).
• Stage 2: The PSUs were divided up into segments (generally city blocks or their equivalent). As with each PSU, sample segments were selected with PPS.
• Stage 3: Households within each segment were listed, and a sample was randomly drawn. In geographic areas where the proportion of age selected for oversampling was high, the probability of selection for those groups was greater than in other areas.
• Stage 4: Individuals were chosen to participate in NHANES II from a list of all persons residing in selected households. Individuals were drawn at random after the screening within designated age subdomains. Approximately one person per sample household was selected for an exam.

What is a Sample Weight?

A sample weight is assigned to each sample person. It is a measure of the number of people in the population represented by that sample person in NHANES II, reflecting the unequal probability of selection, nonresponse adjustment, and adjustment to independent population control totals. When unequal selection probability is applied, as in the NHANES II sample, the sample weights are used to produce an unbiased national estimate. More information about sample weights and how they are created can be found in the Weighting module.

Oversampling

NHANES II was designed to sample larger numbers of certain subgroups of particular public health interest. Certain subgroups were oversampled to increase the reliability and precision of estimates of health status indicators for these population subgroups.