Wash. Rev. Code § 4.22.015
"Fault" includes acts or omissions, including misuse of a product, that are in any measure negligent or reckless toward the person or property of the actor or others, or that subject a person to strict tort liability or liability on a product liability claim. The term also includes breach of warranty, unreasonable assumption of risk, and unreasonable failure to avoid an injury or to mitigate damages. Legal requirements of causal relation apply both to fault as the basis for liability and to contributory fault.
A comparison of fault for any purpose under RCW 4.22.005 through 4.22.060 shall involve consideration of both the nature of the conduct of the parties to the action and the extent of the causal relation between such conduct and the damages.
RCW 4.22.015