318 Management skills Measures of central tendency Measures of central tendency identify the middle or centre of a set of data. There are three types: Arithmetic average or mean ­ the total of items or scores in a set divided by the number of individual items in the set. It may give a distorted picture because of large items at either end of the scale. Median ­ the middle item in a range of items (often used in pay surveys when the arithmetic mean is likely to be distorted). Mode ­ the most commonly occurring item in a set of data. Measures of dispersion It is often useful to measure the extent to which the items in a set are dis- persed or spread over a range of data. This can be done in five ways: By identifying the upper quartile or lower quartile of a range of data. The strict definition of an upper quartile is that it is the value which 25 per cent of the values in the distribution exceed, and the lower quartile is the value below which 25 per cent of the values in a distribution occur. More loosely, especially when looking at pay