A systematic review attempts to identify, appraise and synthesize all the empirical evidence that meets pre-specified eligibility criteria to answer a given research question. Researchers conducting systematic reviews use explicit methods aimed at minimizing bias, in order to produce more reliable findings that can be used to inform decision making. (See Section 1.2 in the Cochrane Handbook for Systematic Reviews of Interventions.)
Cochrane Reviews are systematic reviews of research in healthcare and health policy that are published in the Cochrane Database of Systematic Reviews. There are three types of Cochrane Review:
1. Intervention reviews assess the benefits and harms of interventions used in healthcare and health policy.
2. Diagnostic test accuracy reviews assess how well a diagnostic test performs in diagnosing and detecting a particular disease.
3. Methodology reviews address issues relevant to how systematic reviews and clinical trials are conducted and reported.
Cochrane Reviews base their findings on the results of trials which meet certain quality criteria, since the most reliable studies will provide the best evidence for making decisions about health care. Authors of Cochrane Reviews apply methods which reduce the impact of bias across different parts of the review process, including:
1. Identification of relevant studies from a number of different sources (including unpublished sources);
2. Selection of studies for inclusion and evaluation of their strengths and limitations on the basis of clear, predefined criteria;
3. Systematic collection of data;
4. Appropriate synthesis of data.
These methods are described in detail in the Cochrane Handbook for Systematic Reviews of Interventions and the Cochrane Handbook for Diagnostic Test Accuracy Reviews (in development).
Cochrane Reviews are updated to reflect the findings of new evidence when it becomes available because the results of new studies can change the conclusions of a review. Cochrane Reviews are therefore valuable sources of information for those receiving and providing care, as well as for decision-makers and researchers.
If the results of the individual studies are combined to produce an overall statistic, this is usually called a meta-analysis. Many Cochrane Reviews measure benefits and harms by collecting data from more than one trial, and combining them to generate an average result. This aims to provide a more precise estimate of the effects of an intervention and to reduce uncertainty.
Not every review in the Cochrane Database of Systematic Reviews contains a meta-analysis. This might not be appropriate if the designs of the studies are too different, if the outcomes measured are not sufficiently similar, or if there are concerns about the quality of the studies, for an average result across the studies to be meaningful.
All research should be carried out according to a pre-defined plan. Cochrane researchers use the protocol to describe the proposed approach for a systematic review. It outlines the question that the review authors are addressing, detailing the criteria against which studies will be assessed for inclusion in the review, and describing how the authors will manage the review process. Protocols contain information that defines the health problem and the intervention under investigation, how benefits and harms will be measured, and the type of appropriate study design. The protocol also outlines the process for identifying, assessing, and summarizing studies in the review. By making this information available the protocol is a public record of how the review authors intend to answer their research question.