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 (nth) value. Thus:
To illustrate summation notation, suppose there are five values for a variable X:
X1 = 2 X2 = 0, X3 = -1, X4 = 5, and X5 = 7.
For these data,
In statistics, it is also often necessary to sum the squared values of a ...