[Scientific evidence based medicine: myth and reality of variability in clinical practice and its impact on health outcomes].

Clinical practice is not an exact and reproducible phenomenon since it is carried out in a working framework with a high degree of uncertainty. The different sources of variability in clinical practice are analysed, with special emphasis placed on the quality of the scientific evidence that underlies decision making. On the basis of the change from the traditional model to a practice based on information derived from scientific evidence, different problems are examined in relation to the need for transforming the information into effective action. Finally, with the aim of reducing variability and its harmful impact on health, those areas that should be the object of future analysis and research are enumerated.