Lever

Often when you step on the bathroom scale, what you have in mind is a figure about your size or a wish - oh, say - say ten pounds minus your usual size. Close your eyes, picture a magic figure and nine times out of ten it will be wrong when you look at the display.

Now think about the weighing scale with a lever that goes up and down like a seesaw. Though little known among young people on diet now, this scale was popular some thirty years ago. In case you have difficulty recognising such scale, go to the market and see how the old guys put the fruit or vegetable on one pan and add standard masses on the other pan until the beam is as close to a horizontal line as possible. The same applies to measuring our own body weight on the lever. Step on the base, and set the weight by moving a sliding weight to and from the fulcrum. Hmm... how about 120 pounds? And then the lever arm goes up. You must have taken too many muffins and cakes. So let's slide the weight to the right by one notch, waiting for that lever arm to fall again. Tap, tap. The lever arm moves, but just a little. Fine, let's move it by five notches, tap, tap, tap. The lever arm falls but then overshoots. Try again by going left.

Yeah, that's pretty similar to what we call diagnostic accuracy and calibration in guessing the true answer when doctors see patients. We come up first with an answer with somewhat fair confidence, and struggle around as we move back and forth. You don't have to move the sliding weight too much if you aren't confident in guessing your body weight. Go slow. In other words, zero in on the true answer bit by bit. Consider its similarity with doctors' search for an answer. The less confident a doctor is, the more diagnostic tests he will request. The catch is, doctors move the "sliding weight" but can't see the "lever" in real-world cases. That means shooting one after another, but without being told how far or how close one is from the bull's-eye. Which is why doctors' level of confidence won't be that sensitive to diagnostic accuracy and case difficulty - and definitely worse than the accuracy of guessing your body weight.