The Greek letter sigma (Σ) is a math operator that says, “add up the” whatever comes after it. For example, if you see ΣX, it tells you to add up all the values of X. Remember that X is usually a column heading in statistics, not a single value, as is usually the case in algebra! Only the number immediately after the sigma is summed. If you are to sum the entire expression, then parentheses will be used. Other things can be summed as well using the Σ symbol. In cases where there are ordered pairs of variables labeled X and Y, you will frequently need to compute both the ΣX and ΣY. ΣX means the sum of the X scores, and ΣY means the sum of the Y scores.
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Last Modified: 06/28/2018