80 PART 2 Examining Tools and Processes

the likelihood that this happens, you should try your best to draw a sample that is

representative of the population.

You may be wondering, “What is the best way to draw a sample that is representa-

tive of the background population?” The honest answer is, “It depends on your

resources.” If you are a government agency, you can invest a lot of resources in

conducting representative sampling from a population for your studies. But if you

are a graduate student working on a dissertation, then based on resources availa-

ble, you probably have to settle for a sample that is not as representative of the

population as a government agency could afford. Nevertheless, you can still use

your judgment to make the wisest decisions possible about your sampling approach.

Taking a simple random sample

Taking a simple random sample (SRS) is considered a representative approach to

sampling from a background population. In an SRS, every member of the popula-

tion has an equal chance of being selected randomly and included in the sample.

As an example, recall the printout of the current patient list from a clinic dis-

cussed in the previous section. Considering that list a clinical population, imagine

that you used scissors to cut the list up so that each name was on its own slip of

paper, and then you put all the slips of paper into a hat. If you want to take an SRS

of 20 patients, you could randomly remove 20 names from the hat. The SRS would

be seen as a highly representative sample.

In practice, an SRS is usually taken using a computer so that you can take advan-

tage of a random number generator (RNG) (and do not have to cut up all that paper).

Imagine that the patient list from which you were sampling was not printed on

paper, but was instead stored in a column in a spreadsheet in Microsoft Excel. You

could use the following steps to take an SRS of 20 patients from this list using the

computer:

1.

Create a column containing random numbers.

You could create another column in the spreadsheet called “Random” and

enter the following formula into the top cell in the column: =RAND(). If you drag

that cell down so that the entire column contains this command, you will see

that Excel populates each cell with a random number between 0 and 1. Each

time Excel evaluates, the random number gets recalculated.

2.

Sort the list by the random number column.

3.

Select the top 20 rows from the list.