The Standard Deviation Formula. In this formula, x is the value of the mean, N is the sample size, and x i represents each data value from i=1 to i=N.. The ∑ symbol indicates that you must add up the sum. (x 1 - x) 2 + (x 2 - x) 2 + (x 3 - x) 2 + (x 4 - x) 2 + (x 5 - x) 2. . . + (x N-x) 2.