MEASURES OF DISPERSION OF UNGROUPED DATA Mean Deviation (average absolute deviation) The formula for mean deviation is shown below. Example Mean deviation is not used for higher statistical work. The useful measures of dispersion are variance and standard deviation. Population variance and Standard deviation of ungrouped data Variance determines how the values fluctuate around the mean. The population variance is the mean value of the square of the deviation from the mean as is denoted by the symbol sigma square. The formula is shown below. Another form of the formula which is more convenient to use is as follows: σ 2 = 1 N ∑ X 2 −μ 2 Standard deviation is the square root of the variance One form of the formula is as shown below.