1) It may be inferred that harm suffered by the plaintiff is caused by negligence of the defendant when
 a.the event is of a kind that ordinarily does not occur in the absence of negligence;
 b.other responsible causes, including the conduct of plaintiff and third persons are sufficiently eliminated by the evidence; and
 c.the indicated negligence is within the scope of the defendant’s duty to the plaintiff
2) It is the function of the court to determine whether the inference may be reasonably drawn by the jury, or whether it must be necessarily drawn.
3) It is the function of the jury to determine whether the inference is to be drawn in any case where the different conclusions may be reasonably reached

Three factors in Res Ipsa
i.   Accident causing injury would not have occurred unless someone was negligent Newing v. Cheatham- Plane crashed into mountain evidence proved negligence
ii.  Instrumentality causing injury was in D sole control Larson v. St. Francis Hotel- Chair thrown out of hotel wind was not in D’s control 
iii.  Injurious condition was not due to any vol. action or contrib. on part of P.
Key: the more uncertainty there is to the cause = more likely RIL
Good example of Res Ipsa – Ybarra v. Spangard P. receives damage to shoulder during appendectomy