The Bone & Joint Journal provides the latest evidence to guide the clinical practice of orthopaedic surgeons. The benefits of one intervention compared with another are presented using outcome measures; some may be specific to a limb or joint and some are more general health-related quality of life measures. Readers will be familiar with many of these outcome measures and will be able to judge the relative benefits of different interventions when measured using the same outcome tool; for example, different treatments for pain in the knee measured using a particular knee score. But, how should readers compare outcomes between different clinical areas using different outcome measures? This article explores the use of standardised effect sizes. © 2014 The British Editorial Society of Bone & Joint Surgery.