Sources of evidence within a systematic review

The overwhelming majority of systematic reviews involve therapy. Therefore, randomised controlled clinical trials should be used for systematic reviews of therapy if they are available because they are generally less susceptible to selection and information bias than are other study designs. The quality of included trials is assessed using the criteria that are used to evaluate individual randomised controlled clinical trials. The quality criteria commonly used include:

• concealed random allocation

• groups similar in terms of known prognostic factors

• equal treatment of groups

• accounting for all patients entered into the trial in analysing results (intent-to-treat design).

These are discussed in more detail in Chapter 9.

Systematic reviews of treatment efficacy should always include an assessment of common and serious adverse events as well as efficacy, in order to come to an informed and balanced decision about the utility of a treatment. Randomised controlled trials are rarely a reliable source of identification of adverse reactions, unless the adverse events are very common. Other sources of evidence such as case-control studies, case reports and post-marketing surveillance studies should therefore be examined, as discussed in Chapter 10.

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