There is a hierarchy to the published evidence found. The higher on the Hiearchy of Evidence Pyramid the better the evidence for decision making. THe pyramid includes filtered and unfiltered resources.
Filtered Resources appraise the quality of studies and often make recommendations for practice. Types of filtered sources:
Systematic Reviews / Meta-Analyses (Available in Multiple databases)
Authors of a systematic review ask a specific clinical question, perform a comprehensive literature search, eliminate the poorly done studies and attempt to make practice recommendations based on the well-done studies. A meta-analysis is a systematic review that combines all the results of all the studies into a single statistical analysis of results.