Evidence-Based Practice for Nurses

disease such as asthma may be excluded from the study. The researcher may
believe that having another chronic disease is an extraneous variable that could
affect coping. If researchers choose to set exclusion criteria, the criteria must
be clearly delineated, and researchers must have valid explanations to support
the reasons for the exclusions.

Inclusion and exclusion criteria are also important to appraise when consider-
ing evidence. When criteria are explicit, nurses can determine whether results
are applicable to the clinical practice topic being considered. Clearly defined
inclusion and exclusion criteria tend to improve studies because the precision
of sample selection is enhanced. This precision leads to stronger evidence when
looking at research relevant to clinical practice (Bloom & Trice, 2011).

When samples are not representative of target populations, the potential for
sampling errors and bias is increased. Sampling error occurs when subjects in a
study do not adequately represent the population. In most instances, sampling
error is a result of small sample size that could not adequately represent all of
the elements of the target population. Sampling bias occurs when the sample
includes elements that over- or underrepresent characteristics when compared
to elements in the target population. This is a threat to the external validity
of the study or how the study can be generalized and reported to reflect the
target population.

KEY TERMS

sampling error:

Error resulting

when elements

in the sample do

not adequately

represent the

population

sampling bias: A

threat to external

validity when a

sample includes

elements that over-

or underrepresent

characteristics

when compared

to elements in the

target population

How did you do? 1. b; 2. e; 3. g; 4. a; 5. h; 6. d; 7. c; 8. f

Match the following:

1. Population a. The elements included in a study


2. Target population b. All elements that meet a well-defined
   set of criteria


3. Accessible population c. Degree to which the sample reflects the
   population


4. Sample d. Characteristics that eliminate elements
   from a study


5. Inclusion criteria e. A population that meets sampling criteria


6. Exclusion criteria f. Basic unit of the population


7. Representativeness g. Population from which a researcher can
   sample


8. Element h. Characteristics of elements in a study

TEST YOUR KNOWLEDGE 11-1

11.1 Fundamentals of Sampling 289