The simplest way to indicate the precision of a measurement is by the number of digits that you write down when you record it. In the example above, notice that the measurement was recorded as 25.45 cm - not 25.4 cm, or 25.5 cm, or 25.52 cm. The number of digits that you write down for a measurement is called the number of significant digits (or significant figures) in the measurement. Scientists understand that the last digit (and only the last digit) in a measurement is an estimate. By writing 25.45 cm, you indicate that you are sure that the measurement was between 25.4 cm and 25.5 cm, and you estimate that it was about 5/10 of the way between them.25.45 cm is the best estimate (so far) for the length of a pendulum. We could take a step beyond significant digits and estimate the range of values that we believe the "true length" lies within. In the diagram above, you could certainly tell if the measurement were smaller than about 25.43 cm or larger than 25.47 cm. Therefore, you are confident that the "true value" lies within 0.2 mm (0.02 cm) of 25.45 cm. You can indicate this by writing the measurement as This is why "measurements are not numbers". A physical measurement consists of the best estimate of the "true value" and the range in which the "true value" probably lies. How did I come up with the 0.2 mm range above? Well, by looking at the diagram above, and thinking about it. It seemed like a reasonable value to me. No, there is no set of rules that will tell you what range to use (although there are some ways to get suggestions...). You have to pick a range that seems justifiable to you, and then be ready to justify it.