Whereas the mean deviation is just that - the mean distance of the measurements from the mean, the standard deviation is usually calculated by first finding the variance and then extracting the square root of the variance. The mean deviation and the standard deviation indicate how much the average value in the data set differs from the mean. In a sense, the interquartile range is the range of the middle half of the data. Similar in spirit to the range, but not affected by outliers, is the interquartile range, that is, the difference between the third and first quartiles. It is usually used in conjunction with a measure of central tendency, such as the mean or median, to provide an overall description of a set of data. Of all measures of dispersion, the range is most sensitive to extreme values. A measure of spread, sometimes also called a measure of dispersion, is used to describe the variability in a sample or population.
The range is the distance between the largest and the smallest value in the data set. The simplest of these measures is the range. Measures of dispersion indicate how much the data in a given set of numerical data are spread out. Dividing by the means ( 269.7 m m for length, 100.6 m m for width), the coefficients of variation is actually slightly smaller for length ( 4.9 ) than for width ( 5.2 ), which for most purposes would be a more useful measure of variation. There are different measures of dispersion like the range, the quartile deviation, the mean deviation and the standard deviation. << Chapter 3 Measures of Dispersion : Chapter 4 The scatterness or variation of observations from their average are called the dispersion.
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