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APPRAISAL OF RESEARCH LITERATURE
Let’s assume your searching of the literature has identified several 
relevant primary studies. All the studies use an appropriate study
design for answering your research question. Consideration still
needs to be given to the fact that within any particular design there 
is huge variability between studies with regard to how well they are
conducted. An important issue to consider at this stage is the study’s
validity, in particular internal validity.
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Chapter 8
Internal validity refers to the degree to which the results of a study
are likely to approximate to the ‘truth’ for the circumstances being
studied
4
. Has the study been conducted in such a way that systematic
error (bias) has been minimised? External validity refers to the degree
to which the effects observed in the study are applicable to the outside
world; how generalisable are the results to other circumstances
5
? If
internal validity does not exist, there is little point in considering a
study’s external validity. There are numerous ways in which bias can
affect the internal validity of a study. Some of the main biases
6
are
described in Table 8.4.
There is a plethora of critical appraisal checklists and resources
available. Some guidance on the types of questions to ask
7,8
when
reading research articles is presented below:
• What is the research question?
• Is the study design appropriate?
• How well was the study conducted? Consider issues of:
—
Internal validity
—
Sample size
—
Validity of outcome measures
—
Duration of follow-up
Table 8.3
Example of results obtained when different free-text terms, MeSH terms,
search operators and truncation symbols are used when searching an electronic
database (The Cochrane Library, Issue 4, 2003).