What is the test for establishing a contributory factor in a tort?

Causation in tort is established where the plaintiff proves, on a balance of probabilities, that the defendant caused or materially contributed to the harm. A contributing factor will be considered to be material if it falls outside the de minimus range (Athey v. Leonati, 1996 CanLII 183 (SCC), [1996] 3 S.C.R. 458, 140 D.L.R. (4th) 235 [Athey cited to S.C.R.]). A negligent defendant will not be excused from liability merely because other causal factors were at play in producing the harm. As explained in Mr. Justice Major’s memorable passage in Athey: It is not now necessary, nor has it ever been for the plaintiff to establish that the defendants’ negligence was the sole cause of the injury. There will frequently be a myriad of other background events which were necessary preconditions to the injury occurring...as long as a defendant is part of the cause of an injury, the defendant is liable, even though his [or her] act alone was not enough to create the injury. There is no basis for a reduction of liability because of the existence of other preconditions: defendants remain liable for all injuries caused or contributed to by their negligence. (para. 17)