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Meta-analysis refers to the statistical synthesis of quantitative result= s from two or more studies. The review protocol should state that statistic= al meta-analysis of data will be conducted if appropriate and that if meta-= analysis is not possible, narrative synthesis will be conducted as the prim= ary mechanism of data synthesis. Narrative summary should be included to su= pplement the technical details provided on the process and results even if = meta-analysis is performed and to provide synthesis of data not captur= ed in statistical meta-analysis.

Meta-analysis should be reserved for the results of studies that are con= sidered similar enough from a clinical and methodological point of view (ho= mogeneous studies). If studies are heterogeneous from a clinical or methodo= logical point of view, then it is uncertain if it is appropriate to synthes= ize the respective studies into meta-analysis. Any meta-analysis where stud= ies are heterogeneous from a clinical or methodological point of view will = require substantial justification by the authors. Clinical heterogeneity re= fers to differences between studies with regards the participants, interven= tions, comparators, settings, and outcomes. Methodological heterogeneity re= fers to the study design and the methodological quality of the studies (ris= k of bias). Studies that are similar with regards the participants, interve= ntions, comparators, settings, outcomes, study design, and risk of bias may= be combined in meta-analysis. The judgement that studies are homogeneous e= nough and that it is appropriate to combine the studies in meta-analysis sh= ould be based on the understanding of the review question, the characterist= ics of the studies, and the interpretability of the results. The decision s= hould not be based just on statistical considerations regarding heterogenei= ty (Sutton et al 2000).

The review protocol should specify the appropriate possible, reasonable = details regarding the anticipated (pre-planned) meta-analysis:

- Objectives of the meta-analysis,
- Meta-analysis model (fixed effects model or random effects model) and t= he justification,
- Effect size to be used (OR, RR, etc.),
- Meta-analysis method (Peto method etc.) and justification,
- Statistical testing procedures used for the exploration of statistical = heterogeneity (such as Q Cochran test) and the rules used for the interpret= ation of the results,
- Statistical indicator used for the quantification of statistical hetero=
geneity (such as I
^{2}) and the rules used for the interpretation o= f the results, - Pre-planned sensitivity analyses and their justification, and
- Pre-planned subgroup analyses and their justification.

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