Empirical Rule

The Empirical Rule (68-95-99.7) says that if the population of a statistical data set has a normal distribution ... Statistics For Dummies, 2nd Edition.

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Interquartile Range

The range, inter-quartile range and standard deviation are all measures that indicate the amount of variability within a dataset. The range is the simplest measure of variability to calculate but can be misleading if the dataset contains extreme values.

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Quartiles

The inter-quartile range reduces this problem by considering the variability within the middle 50% of the dataset. The standard deviation is the most robust measure of variability since it takes into account a measure of how every value in the dataset varies from the mean.

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www2.le.ac.uk

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Sum of Squares

Statistics. For partitioning of variance, see Partition of sums of squares; For the "sum of squared deviations", see Least squares; For the "sum of squared differences", see Mean squared error

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The Range

A related measure of variability is called the semi-interquartile range. The semi-interquartile range is defined simply as the interquartile range divided by 2. If a distribution is symmetric, the median plus or minus the semi-interquartile range contains half the scores in the distribution.

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onlinestatbook.com

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Variance

I could only find one result from Yahoo Answers, but the answer wasn't clear on the exact meaning of variability and how it is different from variance.

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english.stackexchange.com

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