CHAPTER 6 Taking All Kinds of Samples 83

Imagine you are surveying a sample of patients about their opinions of waiting

times at a particular emergency department, and you are doing this in the time

window of between 6 p.m. and midnight tonight. To take a systematic sample of

this population, follow these steps:

1.

Select a small number.

This is your starting number. If you select three, this means that — starting at

6 p.m. — the first patient to whom you would offer your survey would be the

third one presenting to the emergency department.

2.

Select another small number.

This is your sampling number. If you select five, then after the first patient to

whom you offered the survey, you would ask every fifth patient presenting

to the emergency department to complete your survey.

3.

Continue sampling until you have the size sample you need (or the time

window expires).

Chapter 4 describes the software G*Power that can be used for making

sample-size calculations.

In systematic sampling, you are technically starting at a random individual, then

selecting every kth member of the population, where k stands for the sampling

number you selected.

Systematic sampling is not representative if there are any time-related cyclic pat-

terns that could confer periodicity onto the underlying data. For example, suppose

that it was known that most pediatric patients present to the emergency depart-

ment between 6 p.m. and 8 p.m. If you chose to collect data during this time win-

dow, even if you used systematic sampling, you would undoubtedly oversample

pediatric patients.

Sampling clusters

Another challenge you may face as a biostatistician when it comes to sampling

from populations occurs when you are studying an environmental exposure. The

term exposure is from epidemiology and refers to a factor hypothesized to have a

causal impact on an outcome (typically a health condition). Examples of environ-

mental exposures that are commonly studied include air pollution emitted from

factories, high levels of contaminants in an urban water system, and environ-

mental pollution and other dangers resulting from a particular event (such as a

natural disaster).