Variance
The mean and the median are two alternative ways of describing the middle value of a sequence, but on their own they tell you very little about the values contained within it. For example, if we know the mean of a sequence of ninety-nine values is 50, we can still say very little about what values the sequence contains.
It may contain all the integers from one to ninety-nine, or forty-nine zeros and fifty ninety-nines. Maybe it contains negative one ninety-eight times and a single five-thousand and forty-eight. Or perhaps all the values are exactly fifty.
The variance of a sequence is its "spread" about the mean, and each of the preceding examples would have a different variance. In mathematical notation, the variance is expressed as:
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