Systematic Reviews and Meta-Analyses
Important medical questions are typically studied more than once, often
by different research teams in different locations.
A systematic review is a comprehensive survey of a topic in which all
of the primary studies of the highest level of evidence have been
systematically identified, appraised and then summarized according to
an explicit and reproducible methodology.
A meta-analysis is a survey in which the results of all of the included
studies are similar enough statistically that the results are combined
and analyzed as if they were one study. In general a good
systematic review or meta-analysis will be a better guide to practice
than an individual article.
Pitfalls specific to meta-analysis include:
- It's rare that the results of the different studies precisely agree,
and often the number of patients in a single study is not large enough
to come up with a decisive conclusion.
- If the authors are interested in supporting a particular conclusion,
they can include studies that support that conclusion and omit studies
that do not. Do the authors explain in their paper exactly on what
basis they included studies , and do their reasons make sense?
- Studies that show some kind of positive effect tend to be published
more often than those that do not. This means that if the authors
include only published studies, several weak positive studies may
seem to add up to a strong positive result. Do weak negative studies
exist? This effect is known as Publication bias.