Because the operation of addition occurs so frequently in statistics, the special symbol ∑ is used to mean “take the sum of.” If there is a set of n values for a variable labeled X, the expression means that these n values are to be added together from the first value to the last (n^{th}) value. Thus:

To illustrate summation notation, suppose there are five values for a variable X:

X

_{1}= 2 X_{2}= 0, X_{3}= -1, X_{4}= 5, and X_{5}= 7.

For these data,

In statistics, it is also often necessary to sum the squared values of a ...

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