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).1 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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