Central principles
 

Approximation

The next major principle of measurement is that nearly all measurements are approximate. If you are measuring length, mass, time or capacity, all you can ever achieve is to make the measurement to the nearest something, depending on the level of accuracy of your measuring device. The reader may have noticed that against the stated mass or volume on many food and drink packages there is a large ‘e’. This is a European symbol indicating that the stated measurement is only an estimate within mandatory limits.

The principle of measuring to the nearest something and the associated language should be introduced to primary-school pupils from the earliest stages. Even when measuring is non-standard units they will encounter this idea, as, for example, when determining that the length of the table is ‘nearly 9 spans’ or ‘about 9 spans’ or ‘between 8 and 9 spans’.