In Search of Evidence

Systematic Reviews and Meta-Analyses
comparable and thus the studies share the same type of organization, target
group, intervention, measurement tool and outcome measures. Critical
examination of the terms and concepts on which the studies are based is also
important. For instance, the term Aspirin is understood in the same way in most
countries and cultures, whereas this is probably not the case with the terms
“leadership” and “participation” (Petticrew & Roberts, 2006). The observation
that studies use the same terms in their stated research question is therefore
insufficient; it is also important to assess whether the studies all understand the
same thing by these terms.
The second aspect relates to the studies' research methodology
(methodological heterogeneity). A combination of observational studies, noncomparative
studies and surveys is methodologically too different to combine
with each other (Hatala, Keitz, Wyer, & Guyatt, 2005). In that case only the
outcomes of studies with the same research design may be combined.
Figure 1
The third aspect relates to the statistical comparability of the outcome of the
studies (statistical heterogeneity). Even if the studies are based on the same
research question and use the same research design, the outcomes cannot