Deviation and variability
The fact that in any set of data the actual values differ from one another, and also from the mean value itself, has been stressed several times in the foregoing pages. A necessary corollary is that for a true and worthwhile understanding of the mean value of a set of data it must be possible to associate that mean easily and readily with some measure of the degree of scatter about that mean. If this can be achieved then the utility of the mean value is greatly increased, the effectiveness of the descriptive role of statistics is improved and many further deductions can be made concerning other properties of the set of data under consideration. Such applications in the field of geography will be presented in succeeding chapters.