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The evolution of evidence hierarchies: what can Bradford Hill's ‘guidelines for causation’ contribute?

2009; SAGE Publishing; Volume: 102; Issue: 5 Linguagem: Inglês

10.1258/jrsm.2009.090020

1758-1095

Jeremy Howick, Paul Glasziou, Jeffrey K Aronson,

Clinical practice guidelines implementation

High quality randomized controlled trials (RCTs) (concealed allocation, relevant groups blinded and sufficiently powered, etc.) will usually provide sufficient evidence to establish that a particular treatment caused an outcome. Yet sufficiently well‐conducted RCTs are rare.1 Trials can be under‐powered,2 or unsuccessfully blinded,3,4 and often suffer from many undetected biases. The results of most RCTs are therefore often insufficient to establish causation. At the same time, RCTs are often not required to establish causation.5 Treatments including the Heimlich manoeuvre, cardiac defibrillation and parachutes to prevent death6 have never been tested in RCTs, yet their effectiveness is surely strongly supported by evidence. Evidence‐grading systems that place randomized trials at the top of a hierarchy7–13 will deliver misleading conclusions in cases where RCTs are insufficient or unnecessary. According to these hierarchies, trails of homeopathy – often generating positive results and generally of higher quality than RCTs of conventional treatments14 – will be considered to provide strong evidence, whereas the evidence base for the Heimlich manoeuvre to unblock airways and parachutes to prevent death will be judged as less strongly supported by evidence. Sir Austin Bradford Hill, in a widely‐cited ‘pre‐EBM’ system for appraising evidence, suggested that several relevant factors must be considered before concluding causation. We investigated and revised the Bradford Hill ‘guidelines for causation’, in order to refine our intuitions about whether to believe that intervention is effective. Our intention is not to debunk previous attempts to grade evidence, but rather to contribute to their natural evolution and development.