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Imagine you are the clinician who is responsible for a youth athletic programme. You know there are effective injury prevention programmes to reduce the risk for ACL injuries; the coach/manager asks you to decide which programme to implement for your team. However, ACL injuries are not as common as other injuries. To help decide whether it is worth making these programmes mandatory for all athletes under your care, you need to know the risk for ACL injury. A recent systematic review of 58 studies from various sports provides some data.1 As you read the systematic review, you note that the authors advise cautious interpretation of the results because of heterogeneity. What is heterogeneity and should you care?
Heterogeneity is a challenging issue in systematic reviews and a key factor in the decision to pool or not to pool the results of available studies. Here we guide readers and authors of systematic reviews on heterogeneity and its sources: clinical and methodological diversity (box 1).2
Definitions of key terms2
Heterogeneity: Genuine variability in the results of individual studies (above that expected by chance). The source of this variability is clinical or methodological diversity in the studies.
Clinical diversity: Between-study differences in participant characteristics, interventions or outcomes.
Methodological diversity: Between-study differences in design and risk of bias.
Bias: Methodological issues that systematically distort results.
Heterogeneity source 1: clinical diversity
Consider the individual studies in the ACL systematic review.1 Would we expect athletes of different ages, who play different sports at different levels to have the same risk for ACL injury? BJSM readers know, either from clinical practice or research, that different factors influence the risk for injury to a greater or lesser extent. Given your knowledge, do the included studies represent athlete groups with inherently different injury risks? Some studies include athletes …
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