Quality assessment of trials

Once studies are identified and prespecified entry criteria met, a last set of quality criteria may be applied. Scales are available, but essentially they rate selection and observation bias (see above). The description of concealment of allocation is central, as this methodological parameter has consistently been shown to be linked with an estimate of effect. If this is poorly reported, the trial is likely to overestimate the effect of the experimental intervention. (5.,25) For trials that describe allocation with nothing more than 'randomized', this single parameter may not be a sensitive measure of quality. A scale addressing both selection and observation bias by rating the description of randomization, blinding, and reasons for people withdrawing, may be more appropriate. (39

A systematic review should prestate the level of quality that is acceptable, or, at the very least, how the data from poor-quality studies are to be managed. Data extraction

Reliable data extraction is important. Just as studies must be selected with due regard for the inclusion of bias, so data must be extracted carefully and reproducibly. Often reviewers ensure maximum reliability by organizing double data extraction by an independent reviewer.

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