Evidence-Based Practice for Nurses

the size of the sample, who will be eligible to be in the study, how individuals

will be selected, and how they will be recruited (Bloom & Trice, 2011). It is not

economical, feasible, or time efficient to include all possible subjects in a study.

An alternative is to identify a select group of subjects that is representative of

all eligible subjects. These individuals constitute the sample.

To obtain the sample needed for the study, researchers must identify the

target population, which is defined as all elements that meet the study criteria.

For example, if a researcher is conducting a study on individuals ages 13–17

who have type 1 diabetes, then every single adolescent who has type 1 diabetes

and who is 13–17 years of age would be included in the target population. The

researcher may choose to narrow the scope of the study by defining the target

population as a more specific group of subjects. For example, the target popu-

lation could be defined as adolescents 13–17 years of age with type 1 diabetes

whose care is managed by a local diabetic clinic. The researcher determines the

target population based on the purpose of the study, the design, and the type

of data collection being planned (Fawcett & Garity, 2009).

After the target population is defined, researchers identify the accessible

population, which is the group of elements to which the researcher has reason-

able access. Typically, the accessible population is a smaller group than the target

population; however, in some cases it may be the same group. In the previous

example of adolescent diabetics who received care in one local clinic, the sample

target population and accessible population may be the same. Regardless,

researchers select subjects from the accessible population (see Figure 11-1).

Learning these terms means using them correctly when appraising research

studies. Frequently, individuals use the words population and sample inter-

changeably, which is incorrect. The term sample is used appropriately when one

KEY TERMS

population: The

entire group of

elements that meet

study inclusion

criteria

elements: Basic

unit of the

population such as

individuals, events,

experiences, or

behaviors

subjects:

Individuals who

participate in

studies, typically

studies using a

quantitative design

sampling plan:

Plan to determine

how the sample

will be selected and

recruited

sample: A

select group of

subjects that is

representative of all

eligible subjects

target population:

All elements that

meet the study

inclusion criteria

accessible

population: The

group of elements

to which the

researcher has

reasonable access

FIGURE 11-1 Relationship of Sample to Population

Sample

Population

Target

population

Accessible

population

286 CHAPTER 11 Using Samples to Provide Evidence