Evidence-Based Practice for Nurses

Exerting control over extraneous variables affecting the DV is one important
factor to ensure valid results. Study validity refers to the ability to accept results
as logical, reasonable, and justifiable based on the evidence presented. Validity
is sometimes described as the truth or accuracy of the results. Evaluating the
quality of quantitative evidence requires nurses to identify potential threats to
validity, evaluate the seriousness of those threats, and determine whether results
are valid for application to patient care. Threats are forces that can change the
results of studies. Campbell and Stanley (1966) identified two major types of
validity: internal and external (see Table 6-1). Internal and external validity
were originally developed to evaluate cause-and-effect relationships, making
them most relevant for experimental designs. However, validity should be con-
sidered with all quantitative designs. It is important when appraising research
articles to identify threats and consider how these threats may have influenced
the relationship between the IV and DV or the interpretation of results.

Internal Validity

Internal validity is the degree to which one can conclude that it was the IV, not
extraneous variables, that produced the change in the DV (Cook & Campbell,
1979). To establish internal validity, researchers must demonstrate that results
obtained were caused by the IV. Seven common threats to internal validity
include selection bias, history, maturation, testing, instrumentation, mortality,
and statistical conclusion validity.

Researchers can combine between-groups and within-groups designs within
the same study to compare the effectiveness of more than one intervention. In
the pain example, the researcher might offer music therapy as an intervention
to one group of subjects and relaxation therapy as another intervention to a
different group of subjects. The researcher could analyze the within-groups
results to determine the level to which each intervention reduced pain, and
the researcher could compare between-groups results to determine whether
there was a difference in the amount of pain relief provided by the music and
relaxation.

6.2 What Is Validity?

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

‹ Define the two types of validity

‹ Describe seven threats to internal validity

‹ Describe five threats to external validity

‹ Identify strategies used to reduce threats to internal and external validity

KEY TERMS

study validity:

Ability to accept

results as logical,

reasonable, and

justifiable based

on the evidence

presented

internal validity:

The degree to

which one can

conclude that

the independent

variable produced

changes in the

dependent variable

6.2 What Is Validity? 155