Whenever a measurement is made there is always some uncertainty about the result. A balance, a measuring cylinder or a stopwatch cannot give the exact true value, so repeated measurements usually give slightly different results.
The spread of results can be shown in a table or as a bar chart of how many times each value was found. Most results sit close to the mean, which is the sum of all the results divided by the number of results. The range is the highest value minus the lowest value.
The range of a set of measurements about the mean is used as a measure of uncertainty. The uncertainty is half the range, written with a plus or minus sign. A smaller range means the results are closer together, or more precise.
Example: four measurements of a mass are 4.8 g, 5.0 g, 5.2 g and 5.0 g. The sum is 20.0 g, so the mean is 5.0 g. The range is 5.2 − 4.8 = 0.4 g, so the uncertainty is half of this, ± 0.2 g. The result is written as 5.0 ± 0.2 g.