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Summation Notation (Sigma Notation): Definition and Use

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Statistics Definitions > Summation Notation

What is Summation Notation?

Summation (Σ) just means to “add up.” For example, let’s say you had 5 items in a data set: 1,2,5,7,9; you can think of these as x-values. If you were asked to add all of the items up in summation notation, you would see:
Σ(x) which equals 1 + 2 + 5 + 7 + 9 = 24.

When using summation notation, X1 means “the first x-value”, X2 means “the second x-value” and so on. For example, let’s say you had a list of weights: 100lb, 150lb, 153lb and 202lb. The weights and their corresponding x-values are:
X1: 100lb
X2: 150lb
X3: 153lb
X4: 202lb

summation notation
The “i=1” at the base of Σ means “start at your first x-value”. This would be X1 (100lb in this example). The “n” at the top of Σ means “end at n”. In statistics, n is the number of items in the data set. So what this summation is asking you to do is “add up all of your x-values from the first to the last.” For this set of data, that would be:
100 lb + 150 lb + 153 lb + 202 lb = 605 lb.

Note: If you see a number above Σ, instead of n, it means to add up to a certain point. For example, a “3” above the Σ means to sum up the the third item (X3) in the set.

Why the difficult notation? Why not just say “add up”? There are cases when you might want to start at a different point in the data set. Although you probably won’t come across these in an elementary statistics class, if you go onto more advanced stats (or calculus), you’ll come across many different variations. So introducing the Σ notation is getting you used to the format, much like x and y is introduced very early on in basic math.

Summation notation is also a shorthand that helps to avoid long equations. For example, take this lengthy expression, where a, b, and c are constants, and X And Y are random variables.
This can be written more succinctly in summation notation as:
summation notation example

A More Complicated Example

One of the most challenging formulas you’ll come across in elementary statistics that involves summation notation is Pearson’s correlation coefficient:

The Pearson correlation coefficient

The Pearson correlation coefficient

There are multiple summations in the formula and although it’s time consuming to solve, it is fairly straightforward if you break it down into steps. Note that there are two summations of X in the formula:
ΣX2, which means to square the x-values and add them all up
(ΣX)2, which means to add up all of the x-values and then square.

Stephanie Glen. "Summation Notation (Sigma Notation): Definition and Use" From Elementary Statistics for the rest of us!

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