Fundamentals of Statistics contains material of various lectures and courses of H. Lohninger on statistics, data analysis and here for more.

Measures of Variation

If you are collecting data on a process, it is important to determine not only the location of the mean, but also to look at the variation within the data. If you are, for example, interpreting the results of a chemical analysis, you may put much more emphasis on the obtained average value if you know that the individual samples vary only very little in comparison to the mean.

In general, the spread of a distribution, both in absolute and in relative terms, is a good measure of the variability (and hence reliability) of the data. There are several ways to specify the variation in the data: