SUMMATION NOTATION or SUMMATION (Σ) is the most common symbol used in statistics.

In statistics, variables are represented by using capital letters.

Example:
If we want to represent the age of 40 students in a class, the age of the first student is represented by X₁, the age of the second student is X₂, the age of the third student is X₃, and so on until X₄₀. The sum of the ages can be expressed in this way:

X₁ + X₂ + X₃ + X₄ + X₅ + … + X₄₀

However, the summation notation can also be used to write the sum of n values or measurements in a simpler manner. The greek letter Σ (sigma) is the symbol for summation.
Thus, the example above, can also be expressed as:

where:
i = index, with range 1 to 40
lower limit = 1
upper limit = 40

note:
* the subscript of the variable is an arbitrary subscript i
* the range of the summation is indicated in the index

More examples on writing summation notation:

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