ABSTRACT
Chapter 5 introduces measures for accuracy (centring) and precision (spread). It also describes the properties of the normal distribution and its use in understanding process variation and capability.
There are three main measures of the central value of a distribution (accuracy). These are the mean (the average value), the median (the middle value), the mode (the most common value). For symmetrical distributions the values for mean, median and mode are identical.
There are two main measures of the spread of a distribution of values (precision). These are the range (the highest minus the lowest) and the standard deviation. The range is limited in use but it is easy to understand. The standard deviation gives a more accurate measure of spread, but takes a little more effort to calculate and understand.
Continuous variables usually form a normal or symmetrical distribution. The normal distribution is explained by using the scale of the standard deviation around the mean. Using the normal distribution, the proportion falling in the ‘tail’ may be used to assess process capability or the amount out-of-specification or to set targets.
The grouping of data results in an increased sensitivity to the detection of change, which is the basis of the mean chart. The range chart may be used to check and control variation.
