86 PART 2 Examining Tools and Processes

stages as part of a strategy to obtain a representative sample. Figure 6-1 provides

a schematic describing the multi-stage sampling in the U.S. surveillance study

mentioned earlier, NHANES.

As shown in Figure 6-1, in NHANES, there are four stages of sampling. In the first

stage, primary sampling units, or PSUs, are randomly selected. The PSUs are made

up of counties, or small groups of counties together. Next, in the second stage,

segments  — which are a block or group of blocks containing a cluster of

households — are randomly selected from the counties sampled in the first stage.

Next, in the third stage, households are randomly selected from segments. Finally,

in stage four, to select each actual community member who will be offered

participation in NHANES, an individual is randomly selected from each house­

hold sampled in the third stage.

That is how a sample of 8,704 individuals participating in NHANES in 2017–2018

was selected to represent the population of the approximately 325 million people

living in the United States at that time. The good news is that biostatisticians work

on teams to develop a multi-stage sampling strategy — no one is expected to set

up something so complicated all by themselves.

FIGURE 6-1:

Example of

multi-stage

sampling from

the National

Health and

Nutrition.

Examination

Survey (NHANES).

© John Wiley & Sons, Inc.