Evidence-Based Practice for Nurses

11.2 Sampling Methods

At the end of this section, you will be able to:

‹ Differentiate between probability and nonprobability samples

‹ Describe various sampling methods

‹ Identify quantitative and qualitative research sampling strategies

Once inclusion criteria have been determined, researchers identify a method to

select subjects for the sample. Researchers can use several sampling methods

to select subjects. These methods are divided into two categories: probability

and nonprobability sampling.

Probability Sampling Methods

When researchers use probability sampling, every element in the accessible

population has an equal chance of being selected for inclusion in the study. Three

conditions must be met in probability sampling. First, an accessible population

must be identifiable. Next, the researcher must create a sampling frame; that

is, a list of all possible elements in the accessible population. Finally, random

selection must be used to choose elements from the sampling frame. The use of

randomization is important because it reduces the threat of selection bias. The

probability that the characteristics of elements in the sample will be similar to

elements of the population is increased (Graff, 2014; Stevens, 2011). There are

four probability sampling methods: simple random sampling, stratified random

sampling, cluster sampling, and systematic random sampling.

Each of the four probability sampling methods has advantages and dis-

advantages. Researchers should select a method based on the purpose of the

research, the research question, and the research design. Table 11-1 provides

an overview of the assumptions, potential bias, and representativeness of prob-

ability sampling methods.

Simple Random Sampling

Simple random sampling involves randomly selecting elements from the acces-

sible population and is considered by some authorities to be the most effective

method to obtain a representative sample. If the accessible population is small,

simple random sampling may be as easy as drawing names. For example, if

the accessible population is 25 nursing students and the researcher needs 10

students for the study, the name of each student could be placed in a container.

The first name is drawn and recorded as a potential participant. The name is

KEY TERMS

probability

sampling:

Sampling method

in which elements

in the accessible

population have

an equal chance

of being selected

for inclusion in the

study

sampling frame: A

list of all possible

elements in

the accessible

population

randomization:

The selection,

assignment, or

arrangement of

elements by chance

simple random

sampling:

Randomly selecting

elements from

the accessible

population

290 CHAPTER 11 Using Samples to Provide Evidence