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:

1. 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.
2. 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?
3. 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.