Case series

Case series are used to generate hypotheses and to establish effect sizes

Case series generate the lowest level of observational evidence. In a case series design, a series of a specific cases or occurrences of a given disease state are chosen by the researcher and the demographic, clinical, and prognostic characteristics are analyzed. No causal inferences can be made with case series. Case series are useful for hypothesis and effect size generation.  
Case series are used to establish effect sizes and generate hypotheses.
Case series are the lowest level of empirical evidence. Basic and preclinical science, while critical to scientific discovery, often falls into this category.

Case series evidence is considered the lowest form of clinical evidence.
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