5.2.1. Risk of bias assessment of individual studies should be done using a validated assessment tool

For the quality assessment of individual studies, specific tools were selected by the KCE.

There is no agreed KCE tool for observational studies. At this moment, we limit ourselves to a number of elements that need to be verified when looking at observational studies. There are a large number of assessment tools, but in the scientific community there is considerable disagreement on what items really matter. Moreover, observational studies are way more diverse then RCTs.

Study limitations in observational studies as identified by GRADE are:

• Failure to develop and apply appropriate eligibility criteria (inclusion of control population);
• Under- or overmatching in case-control studies;
• Selection of exposed and unexposed in cohort studies from different populations;
• Flawed measurement of both exposure and outcome;
• Differences in measurement of exposure (e.g., recall bias in case-control studies);
• Differential surveillance for outcome in exposed and unexposed in cohort studies;
• Failure to adequately control confounding;
• Failure of accurate measurement of all known prognostic factors;
• Failure to match for prognostic factors and/or lack of adjustment in statistical analysis;
• Incomplete follow-up.