Find the arithmetical average (mean) for a set of data.
(a) Know that the mean is one sort of average (the most common)
(b) Know that the mean is worked out by adding up the items, and dividing by the number of items
(c) Understand that the mean can give a 'distorted average' if one or two values are much higher or lower than the other values, e.g. the mean salary or wage in a company can be distorted by one or two very high salaries