One of the first things to understand when we're looking at literature of any kind is the levels of evidence and where our particular piece of literature falls.
Systematic reviews and Meta-Analysis are considered secondary sources as opposed to primary sources because they have filtered out extraneous research in order to evaluate the highest quality research to answer a particular question.
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Produced by Jan Glover, David Izzo, Karen Odato and Lei Wang.)
Study designs are placed into a hierarchy based on their reliability (probability of bias) known as the Levels of Evidence.
The Levels of Evidence are often represented by a pyramid, with the highest levels of evidence (least common) near the top, and the lowest levels of evidence (most common) near the bottom. The pyramid above is based on the Oxford Centre for Evidence Based Medicine's rating system for therapeutic studies, but a variety of rating systems exist.