In a res ipsa loquitur case the injured party is deemed in no position to explain the cause, while the party charged may begin a position to show himself free from negligence. If the plaintiff has equal or superior means of information, the doctrine does not apply. The question is really one of duty on the part of the defendant. Res ipsa loquitur leads only to a possible (not mandatory) inference that the defendant has not complied with his duty to use skill and care, and is not in itself proof that he was under a specific duty. This question of duty often arises in cases of medical malpractice.
Howard M. Rosen, Defense against Res Ipsa in Medical Malpractice, 13 Clev.-Marshall L. Rev. 128 (1964)