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Part 20: On Odds and Risk Ratios

  • Author Footnotes
    1 Saloni Shah, MD, is a resident in the department of surgery, division of emergency medicine, at the University of Utah's School of Medicine in Salt Lake City, UT.
    Saloni Shah
    Footnotes
    1 Saloni Shah, MD, is a resident in the department of surgery, division of emergency medicine, at the University of Utah's School of Medicine in Salt Lake City, UT.
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  • Author Footnotes
    2 Scott Youngquist, MD, MSc, is an assistant professor in the department and xx at the Air Medical Research Institute in Salt Lake City, UT.
    Scott Youngquist
    Footnotes
    2 Scott Youngquist, MD, MSc, is an assistant professor in the department and xx at the Air Medical Research Institute in Salt Lake City, UT.
    Search for articles by this author
  • Author Footnotes
    1 Saloni Shah, MD, is a resident in the department of surgery, division of emergency medicine, at the University of Utah's School of Medicine in Salt Lake City, UT.
    2 Scott Youngquist, MD, MSc, is an assistant professor in the department and xx at the Air Medical Research Institute in Salt Lake City, UT.
      A recent research article reported as its primary effect measure an adjusted odds ratio (OR) of 0.61 for the association between death from trauma and transport to hospital by helicopter emergency medical services (HEMS) versus ground emergency medical services (GEMS).
      • Sullivent EE
      • Faul M
      • Wald MM
      Reduced mortality in injured adults transported by helicopter emergency medical services.
      Most people understand that an odds ratio > 1, or unity, denotes a positive association between the risk factor and outcome. When the odds ratio is < 1, most people understand that an inverse association is being reported. Assuming no significant residual systematic bias or random error, we conclude that, based on this analysis, HEMS transport was associated, on average, with less death after traumatic injury than transport by GEMS. However, beyond that, much confusion exists. Should we say HEMS transport was associated with less odds of death? Less risk of death? Less chance of death? In this review, we discuss the use of the OR and how it is calculated, and we give suggestions for its interpretation.
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      References

        • Sullivent EE
        • Faul M
        • Wald MM
        Reduced mortality in injured adults transported by helicopter emergency medical services.
        Prehosp Emerg Care. 2011; 15: 295-302
        • Rothman KJ
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        • Lash TL
        Modern epidemiology. 3rd ed. Wolters Kluwer, Lippincott Williams & Wilkins, Philadelphia, PA2008
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        Advanced statistics: up with odds ratios! A case for odds ratios when outcomes are common.
        Acad Emerg Med. 2002; 9: 1430-1434