Factual causation in medical negligence

The conventional approach to causation in negligence is the "but for" test, decided on the balance of probabilities. Even when supplemented by the "material contribution" principle, satisfying the onus of proof of causation can be an insuperable obstacle for plaintiffs, particularly in medical cases. Yet, having found a breach of duty, a court's sympathies may gravitate toward the plaintiff at this point in the case. Accordingly, courts have sometimes accepted a relaxation of strict causation principles. The judicial devices are described: a special principle of causation in particular duties of care; a shifting burden of proof; "bridging the evidentiary gap" by drawing a robust inference of causation; treating a material increase in risk as sufficient proof of causation; and permitting causation to be established on the basis of the loss of a material chance of achieving a better outcome and discounting damages. In Accident Compensation Corp v Ambros [2007] NZCA 304 the New Zealand Court of Appeal recognised the need for a legal device to ameliorate the injustice sometimes caused by the strict rules of causation, and preferred the "inferential reasoning" approach favoured by the Canadian common law for use in the context of the accident compensation scheme. It is hoped that the New Zealand Supreme Court approves Ambros if the opportunity arises.