De praktijk van systematische reviews. VII. Het combineren van de resultaten van observationele onderzoeken

- Knowledge concerning aetiologic hypotheses can be obtained through systematic reviews of observational studies.

- In observational studies, heterogeneity between studies is expected, because of differences in study design and analysis.

- Data from observational studies have to be made comparable before pooling of results from component studies is possible.

- In this, definition of exposure and disease and adjustment for confounding have to be taken into account.

- Three methods to deal with heterogeneity are presented: ignoring (fixed effects model), modelling (random effects model) and exploring (meta regression).

- Any combination of data in meta-analysis of observational studies has to be based on statistical, methodological and clinical considerations. The same considerations are applied for the choice which method is used.