“To see why noise can be a problem, return to the medical context. Suppose that doctors order a large number of tests in the morning, but that in the afternoon, they ask patients to go home and take aspirin. Or suppose that when doctors are in a good mood, they make very different decisions from those they make when they are grumpy. If so, doctors might not show a systemic bias of any kind. But they will be noisy, and the noise will be produce plenty of mistaken decisions.”