Figure 2.

Circle of methods. Experimental methods that test specifically for efficacy (upper half of the circle) have to be complemented by observational, non-experimental methods (lower half of the circle) that are more descriptive in nature and describe real-life effects and applicability. The latter can range from retrospective audit studies, prospective case series to one armed to multiple armed cohort studies. Matched pairs studies can be conducted as experimental studies, by forming first pairs and then randomizing them, or as quasi-experimental studies by forming pairs from naturally occurring cohorts according to matching criteria. Shading indicates the complementarity of experimental and quasi-experimental methods, of internal and external validity.

Walach et al. BMC Medical Research Methodology 2006 6:29   doi:10.1186/1471-2288-6-29
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