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→Variance and standard deviation
===Variance and standard deviation===
===Variance and standard deviation===
The variance of a set of data is calculated by adding together the squared differences of each value from the mean and dividing this by the number of observations. The ''square'' of each difference is used because if the difference itself were used, the values higher than the mean and the values lower than the mean would cancel each other out, meaning that the resulting number would be zero. However, as the squares are used, the variance is expressed in terms of the square of the units of measurement (for example, the variance of the weights of a sample of animals may be 25kg<sup>2</sup>. As this is not easy to relate back to the original units of measurement, the ''square root'' of the variance is often used - which is known as the '''standard deviation'''. The variance and standard deviation should generally only be used in cases where the mean is used as a measure of central tendency, as they relate to this mean in their calculation. As for the mean, they are also affected by the presence of outliers.
The variance of a set of data is calculated by adding together the squared differences of each value from the mean and dividing this by the number of observations minus one (= degrees of freedom). The ''square'' of each difference is used because if the difference itself were used, the values higher than the mean and the values lower than the mean would cancel each other out, meaning that the resulting number would be zero. However, as the squares are used, the variance is expressed in terms of the square of the units of measurement (for example, the variance of the weights of a sample of animals may be 25kg<sup>2</sup>. As this is not easy to relate back to the original units of measurement, the ''square root'' of the variance is often used - which is known as the '''standard deviation'''. The variance and standard deviation should generally only be used in cases where the mean is used as a measure of central tendency, as they relate to this mean in their calculation. As for the mean, they are also affected by the presence of outliers.
===Interquartile range===
===Interquartile range===