To compute a standard deviation for a population: compute the sums of the squared deviations subtract the mean from each score to get deviation scores (x-u) square the deviation scores (x-u)^2 sum the deviation scores E(x-u)^2 compare variance (standard deviation^2) divide the sum of the squared deviations by the number of scores E(x-u)^2 / N take the square root of the variance to get to the standard deviation take the square root of E(x-u)^2 / N Computing standard deviation for a sample: similar except SS (sums of squares) is divided by N-1 and the sample mean (M) is used instead of the population mean (u). SD = square root of SS / N-1