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Searching for the right evidence: how to answer your clinical questions using the 6S hierarchy
  1. Donna Windish
  1. Department of Internal Medicine, Yale University School of Medicine, New Haven, Connecticut, USA
  1. Correspondence to Donna Windish
    Department of Internal Medicine, Yale Primary Care Residency Program, Yale University School of Medicine, 64 Robbins Street, Waterbury, New Haven, CT 06708, USA; donna.windish{at}

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Asking and answering clinical questions during daily practice can be challenging and time consuming. Knowing the resources available to answer a specific clinical question can lead to a more efficient and effective search strategy and thus, to a more applicable answer based on the levels of evidence available. This primer reviews how to search for the right evidence using a specified hierarchy and provides examples of pre-appraised resources with corresponding websites to help with your search.

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Table 1

Clinical questions and available resources associated with the 6S model of evidence


The readers of our journal most likely have busy clinical, administrative and/or teaching roles. As such, time is of the essence when thinking about answering clinical questions that arise in patient care. DiCenso and colleagues recently published a hierarchy of pre-appraised evidence called the 6S model.1 This model denotes in a pyramidal fashion the six levels of evidence available in clinical decision making. It starts with the largest resource available at the base of the pyramid with individual original studies. It then moves upward through synopses of studies, syntheses, synopses of syntheses, summaries and ends with the most specific form of evidence at the apex, with the “systems” layer. This article is a primer on how to use the 6S model when searching for and assessing the highest quality of evidence. The text and Table provide a summary view of clinical questions associated with each stage of the hierarchy, the strengths and weaknesses …

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  • Competing interests None.

  • Correction notice This article has been corrected since it was published Online First. The Introduction section has been added into the article.