"The scale is the only objective measure"

You've probably tried the alternatives and found them wanting. The mirror lies according to the light and your mood. Clothes stretch. How you feel is hopeless as a gauge. The scale is the one thing that gives the same answer whoever's asking, and that's why you trust it. Objective, you think, is the same as meaningful.

It isn't, and the distinction is the whole of this chapter. A measurement can be perfectly accurate and still tell you nothing about the thing you care about, if the thing you care about isn't what it measures on that occasion. Your scale, to the nearest hundred grams, is probably right. What it's right about is the total load on the mat at ten past seven, which chapter 3 took apart. It's an objective measure of a quantity that swings for a dozen reasons, and only one of those reasons is the one you're asking about.

Here's the one analogy I'll use in this chapter. If you wanted to know whether the sea level was rising, you wouldn't stand on the beach and read the waterline once a morning, because the tide would make a fool of you twice a day. You'd average over months and look at the drift underneath. The tide is objective too. It just isn't the answer to the question.

A single reading, then, has no meaning on its own. It only means something as one point among many, once you can see which way the points are going. Researchers call that separating signal from noise, and the words are plain enough to keep. The signal is the slow change in fat, if there is one. The noise is everything in chapter 3. On one morning, the noise is bigger than the signal could possibly be, so the reading is all noise as far as your question goes. Over four weeks, the noise averages out, because water comes and goes and the month completes, and whatever's left is signal.

The scale can give you the signal. It's the only household object that can. But it can only give it to you as a line, and you've been asking it for a point. The trials in chapter 6 that found weighing helpful were, almost without exception, trials in which people plotted the number. The objectivity was the same in your bathroom and in their study. The difference was that they let it accumulate into something that meant something, and you read it fresh each morning and react.

So keep the trust. The scale has earned it. Just move the trust from the number to the line, and stop asking the point for an answer it hasn't got.