In Search of Evidence

Systematic Reviews and Meta-Analyses
experiment) and observational studies such as cohort and case-control studies
(see Table 1). Surveys and case studies are regarded as research designs with
the greatest chance of bias in their outcome and therefore come low down in
the hierarchy. Right at the bottom are claims that are based solely on experts'
personal opinions. Systematic reviews with a meta-analysis of randomized,
controlled trials are therefore regarded as the highest level of evidence.
Table 1: Levels of evidence*
Level Study design Conclusion
A+ Systematic reviews of level A studies
A
Systematic reviews of level B studies
Randomized controlled trials (RCT)
It is shown that ...
B
Systematic reviews of level C studies
Controlled trials without randomization, Cohort studies and Case control
studies
It is likely that …
C
D
Controlled studies without randomization and without a pretest,
uncontrolled studies with a pretest
Uncontrolled studies without a pretest, cross-sectional surveys and casestudies
There are signs that
…
D- Expert opinions
Experts are of the
opinion that …
*Based on the Levels of Evidence of the Oxford Centre for Evidence-based Medicine and the classification of
Campbell (Shadish, Cook and Campbell, 2002, Experimental and Quasi-Experimental Designs for
Generalized Causal Inference)
Hierarchy of evidence: which research type for which question?
In the field of management RCTs are almost non-existant. As a result
systematic reviews will mainly be carried out with reference to non-randomized
and observational research, case studies and surveys. Although the internal
validity of this type of research is lower than that of randomized research, this
does not mean that such forms of research are less usable. Moreover, the levels