Once you've conducted a comprehensive literature search and compiled a database of potentially useful studies, you now need to critically appraise the literature. You need to draw up appropriate criteria to decide which studies to include or exclude from your review, based on an unbiased appraisal of their strengths and limitations.
Modules 4-5 of Cochrane Interactive Learning have information on selecting studies, including how to assess the risk of bias. The Library is subscribing to Cochrane Interactive Learning.
How to select studies for inclusion, systematically collecting different data types that meet criteria.
What bias is, how to assess the risk of bias in randomised trials in different sources.
The University of Exeter provides a short guide on how to critically appraise different types of medical and health literature.
How to Read a Paper by 