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[Observational studies in the era of evidence based medicine: short review on their relevance, taxonomy and designs].

In this review of the literature, we distinguish between experimental and observational studies, highlighting the importance that the later have gained in the era of evidence-based medicine. We further analyze the value of observational studies in light of experimental studies. We present a taxonomy for observational studies based on units of observation and measurement (cross-sectional or longitudinal). We distinguish between descriptive studies and analytical studies. Then, and given its specificity, we define and present a classification for ecological studies. We define and consider the advantages and disadvantages of cross-sectional, case control and cohort studies. We analyze the strength of the evidence given by each study design. We finished by examining what should guide the choice of a study design.