82 PART 2 Examining Tools and Processes

In our scenario, if you choose to draw a stratified sample by age groups, you would

first have to separate the list into a pediatric list and a list of everyone else. Then,

you could take an SRS from each. Because you are concerned about each stratum,

you could make a rule that even though pediatric patients make up only 10 percent

of the background population, you want them to make up 50 percent of your

sample. If you did that, then when you took your SRS, you would oversample from

the pediatric list and select 10, while also taking an SRS of 10 from the list of

everyone else.

Drawing a stratified sample requires you to weight your overall estimate, or else

it will be biased. As an example, imagine that 15 percent of pediatric patients had

an oral health condition, and 50 percent of the rest of the patients had an oral

health condition. In a stratified sample of 20 patients where you draw 10 from the

pediatric population and 10 from the rest of the population, because the pediatric

population is oversampled (because they only make up 10 percent of the back-

ground population but make up 50 percent of our sample), if weights are not

applied, the estimate of the percentage of the population with an oral health con-

dition would be artificially reduced. That is why it is necessary to apply weights to

overall estimates derived from a stratified sample.

If you are familiar with large epidemiologic surveillance studies such as the

National Health and Nutrition Examination Survey (NHANES) in the United States,

you may be aware that extremely complex stratified sampling is used in the design

and execution of such studies. Stratified sampling in these studies is unlike the

simple example described earlier, where the stratification involves only two age

groups. In surveillance studies like NHANES, there may be stratified sampling

based on many characteristics, including age, gender, and location of residence. If

you need to select factors on which to stratify, trying looking at what factors were

used for stratification in historical studies of the same population. The kind of

stratified sampling used in large-scale surveillance studies is reviewed later in

this chapter in the section “Sampling in multiple stages.”

Engaging in systematic sampling

Earlier you considered a scenario where a clinic had a printed list of the entire

population of patients from which an SRS could be drawn. But what if you want to

sample from the population of patients who present to a particular emergency

department tonight between 6 p.m. and midnight? There is no convenient list

from which to draw such a sample. In a scenario like this, even though you can’t

draw an SRS, you want to use a system for obtaining a sample such that it would

be representative of the underlying population. To do that, you could use system-

atic sampling.