Why Median Severity and Ordinal Scale Severity Values should not be used for Injury Burden Results: A Critical Review

Abstract

Injury burden is a composite measure of injury incidence and mean severity; this parameter has been reported as an output measure from injury surveillance studies in rugby for over 20 years. The benefits of reporting injury burden results have, more recently, been recognised in other sports. This wider use of injury burden as an output measure from injury surveillance studies has, however, highlighted misunderstandings about how to calculate, present and interpret injury burden data. The aim of this critical review is to explain why median severity and ordinal severity scales should not be used to calculate and report injury burden results in injury surveillance studies. Equations are presented to show how injury burden results should be calculated, and graphs and tables are presented to explain the errors that are introduced when median severity and ordinal scales of severity are used instead of mean severity. This critical review is intended to highlight the correct procedures for calculating, reporting and interpreting injury burden results in order to avoid incorrect results, conclusions and injury prevention recommendations being published.

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“…Severity was reported as the total number of days lost together with median days lost and quartiles, and in the following categories: 0 days, 1–3 days, 4–7 days, 8–28 days, 29–90 days, 91–180 days and >180 days 23. Injury burden was reported as days lost per 1000 hours of exposure and illustrated in risk matrices with incidence and mean severity 25. Illness incidence was reported as the number of illnesses per player per 365 days.…”

“…23 Injury burden was reported as days lost per 1000 hours of exposure and illustrated in risk matrices with incidence and mean severity. 25 Illness incidence was reported as the number of illnesses per player per 365 days. We compared overall incidence 26 between the preseason and competitive seasons, and the injury incidence in match play versus football training with the significance level set at p<0.05.…”

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“…While the value of injury burden as an output measure from injury surveillance studies is evident, there appears to be some confusion in the literature regarding its calculation. For instance, some authors have used median severity to calculate injury burden rather than mean severity, as discussed in a recent critical review 4. In addition, there appears to be no clear guidance within the sports medicine literature regarding the most appropriate way to calculate confidence intervals (CIs) for this metric.…”

“…For instance, some authors have used median severity to calculate injury burden rather than mean severity, as discussed in a recent critical review. 4 In addition, there appears to be no clear guidance within the sports medicine literature regarding the most appropriate way to calculate confidence intervals (CIs) for this metric.…”