How finely to measure

At the clinic, a newborn is weighed to the gram, and the nurse writes every one of them down. Twenty years later the same person steps on a bathroom scale that only shows whole kilos, and that is plenty.

Same body, same question, very different precision. Both are right, because someone decided what the number was for.

A few grams matter for a newborn, because a small change is a big signal. For an adult they are noise, and a scale that showed them would cause worry over a glass of water.

Most of the measuring we do never gets that decision. We inherit a scale and use it.

A small shop asks customers to rate their visit out of a hundred. Nobody can tell a seventy-one from a seventy-four, including the customer who picked it. Three faces, pleased, neutral and unhappy, would tell the owner more, and faster.

The opposite mistake is just as common. A seller who looks at orders once a week, in round numbers, misses the Tuesday a courier started losing parcels. The detail was there. The instrument was too coarse to show it.

Getting this right is not about more precision or less. It is about asking, before the number exists, what decision it will feed and how small a change ought to make you act. Too coarse and you are blind. Too fine and you chase noise and call it rigour.

The question takes a minute. Hardly anyone asks it, which is why so many of the scales we rely on are ones we never chose.