Law and Epidemiological Evidence: Double, Toil and Trouble

In Sienkiewicz v Greif (UK) Limited, the Supreme Court discussed a doubles-the-risk test based on epidemiological studies for the proof of individual causation in toxic tort litigation in the United Kingdom. The issue was obiter in the Appeal. Differing views were expressed and the speeches cannot be interpreted as rejecting the test as a matter of law. Unsurprisingly, therefore, reference continues to be made to the test and the analogous argument that causation can be proved by a statistical likelihood of a better outcome in the absence of breach. It is generally accepted that risk-based epidemiological evidence is admissible in litigation. This raises the question of the continued forensic role of such evidence in English common law. We use a case study with variations to indicate a range of issues that may arise in the application of epidemiological evidence. The issues are multifaceted and demonstrate why a simple formulaic rule based on doubling of the risk (‘a relative risk of two’) could never work. An arbitrary cut-off at a relative risk of two would lead to injustice. Whilst the epidemiological evidence is telling us something of relevance, it does not answer all the questions that are specific to a particular case at a particular moment in time. A better understanding of epidemiological evidence and how it can be applied in individual cases will assist, but it is reasonable to anticipate that considerable controversy will persist in clinical negligence and toxic tort litigation. For that reason, we propose a structured approach to the assessment and use of epidemiological evidence in litigation. This approach may assist decision-makers and others as they navigate the current muddles and misconceptions that surround the forensic role of such evidence.