# Systematic sampling

This, the most widely used form of cluster sampling, will be illustrated by its application to a frequently occurring problem situation that is, a researcher has a complete list from which probability sample is to be selected.

Example: Assume one wishes to study dentists’ attitudes toward dental insurance and decides to sample 20 dentists from a list of 100 dentists. One way of doing this is as follows:

1. Draw a random number between 1 and 5. Assume the number chosen is 2.
2. Include in the sample the dentists numbered 2, 7, 12, 17, 22, —-97. That is, starting with number 2, take every fifth number.

The above is an illustration of systematic sampling. That this is a particular kind of cluster sampling is readily seen if all possible samples produced by this procedure are considered.

Selecting a Systematic Sample:

A systematic sample can be selected from a list of universe items using a four step procedure.

1. The first step is to determine the total number of items in the universe. Divide this figure by the desired sample size. The result is called the sampling interval.

Sampling interval = number of universe items / desired sample size

2. Select a random number between 1 and the sampling interval figure. This identifies the first element on the universe list to be included in the sample.
3. Add the sampling interval to the random number selected in step 2. The total represents the second element on the universe list to be included in the sample.
4. Continue adding the sampling interval to each total to create a new total. Each new total represents another element on the universe list to be included in the sample.

Example: To illustrate, reconsider the problem of choosing a sample of n = 216 from the hotel chain’s list of N= 21,476 prospects. Simple random sampling is a perfectly valid way to do this. However, it is much simpler to use systematic sampling. The sampling interval is 21,476/ 216, or about 99. Draw a random number between 01 and 99. Assume the random number is 14, then the sample will consist of the universe list items numbered 14, 113, 212, 311 and so on.