In the late 1860s, Sir Francis Galton formulated the law of deviation from an average, which has become one of the most useful statistical measures, known as the standard deviation, or SD as most often abbreviated. The standard deviation statistic is one way to describe the results of a set of measurements and, at a glance, it can provide a comprehensive understanding of the characteristics of the data set. Examples of some of the more familiar and easily calculated descriptors of a sample are the range, the median, and the mean of a set of data. The range provides the extent of the variation of the data, providing the highest and lowest scores but revealing nothing about the pattern of the data. And, either or ...

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