In order to make formulas more general, variables can be used with the summation notation. For example,

means to sum up values of X from 1 to N where N can be any number but usually indicates the sample size.

Often an abbreviated form of the summation notation is used. For example, ΣX means to sum all the values of X. When only a subset of the values of X are to be summed then the full version is required. Thus, the sum of all elements of X except the first and the last (the N'th) would be indicated as:

which would be read as the sum of X with i going from 2 to N-1. Some formulas require that each number be squared before the numbers are summed. This is indicated by:

and is equal to 7² + 6² + 5² + 8² = 174.