Readers will naturally wish to know how good the reviewed research is and why you have excluded some studies that address the question at issue. In both situations you need to explain your judgments, which will usually be based on your assessment of study quality and applicability.

The process will usually need to be done in two stages; firstly, an initial screen for basic eligibility criteria and secondly, a detailed appraisal of quality. The eligibility screen might ask whether the study addresses the question and achieves some minimal quality criteria. For example, for an intervention question this might be evidence of a control group. This process is outlined in Figure 3.1.

Standardizing the appraisal

Providing an explicit and standardized appraisal of the studies that have been identified is important for two reasons. Firstly, a systematic review should try to base its conclusions on the highest-quality evidence available. To do this requires a valid and standardized procedure to select from the large pool of studies identified so that only the relevant and acceptable quality studies are included in the review. Secondly, it is important to convey to the reader the quality of the studies included as this indicates the strength of evidence for any recommendation made.

What study features should be assessed?

Overall, the study features that are most important to assess are those that involve selection and measurement bias, confounding and follow-up of participants. In Part 2 these features are examined for each question type under the following headings:

1. Has selection bias (including allocation bias in randomized controlled trials (RCTs)) been minimized?

2. Have adequate adjustments been made for residual confounding?

3. Have the final outcomes been adequately ascertained?

4. Has measurement or misclassification bias been minimized?