Summation Notation (3 of 4)

---

The abbreviated version is simply: ΣX². It is very important to note that it makes a big difference whether the numbers are squared first and then summed or summed first and then squared. The symbol (ΣX)² indicates that the numbers should be summed first and then squared. For the present example, this equals:

(7 + 6 + 5 + 8)² = 26² = 676. This, of course, is quite different from 174.

Sometimes a formula requires that the sum of cross products be computed. For instance, if 3 subjects were each tested twice, they might each have a score on X and on Y.

Subject	X	Y
1	2	3
2	1	6
3	4	5

The sum of cross products (2 x 3) + (1 x 6) + (4 x 5) = 32 can be represented in summation notation simply as: ΣXY.