Variation: part two

摘要

The measurements of variation process can vary in two different ways: in their centre and their spread, as illustrated in figure 1. The centre - also called accuracy or central tendency - of a process, is the degree to which measurements gather around a target value. The spread - also called dispersion or precision - of the process is the degree of scatter of its output values. To measure the centring of a process requires that the centre point of the set of results be identified. The accuracy of the process can then be determined by comparing it with target values. There are three ways of measuring this centre point: the mean (or average), the median and the mode (see figure 2). The most common way of measuring the centre point of a set of measurements is with the average, or mean, ie the sum of all measurements divided by the total number of measurements.