The Greek letter Σ (a capital sigma) is used to designate summation. For example, suppose an experimenter measured the performance of four subjects on a memory task. Subject 1's score will be referred to as X_{ 1 } , Subject 2's as X_{ 2 }, and so on. The scores are shown below:
Subject 
Score 

1 
X_{1} 
7 
2 
X_{2} 
6 
3 
X_{3} 
5 
4 
X_{4}

8 
The way to use the summation sign to indicate the sum of all four X's
is:
This notation is read as follows:
Sum the values of X from X_{1} through
X_{4} .
The index i (shown just under the Σ sign) indicates which
values of X are to be summed. The index i takes on values beginning with the
value to the right of the "=" sign
(1 in this case) and continues sequentially until it reaches the value
above the Σ sign (4 in this case). Therefore
i takes on the values 1, 2, 3, and 4 and the values of X_{1},
X_{2}, X_{3}, and X_{4} are summed (7 + 6 + 5
+ 8 = 26).