What is the test for causation in the professional medical malpractice context?

Causation in a professional malpractice context means establishing by a preponderance of the evidence that, but for the negligence, "the plaintiff would have obtained a more favorable judgment or settlement in the action in which the malpractice allegedly occurred. The purpose of this requirement, which has been in use for more than 120 years, is to safeguard against speculative and conjectural claims." ( Viner v. Sweet (2003) 30 Cal.4th 1232, 1241, 135 Cal.Rptr.2d 629, 70 P.3d 1046.)