To calculate a Z score, start by calculating the mean, or average, of your data set. Then, subtract the mean from each number in the data set, square the differences, and add them all together. Next, divide that number by n minus 1, where n equals how many numbers are in the sample, to get the variance. Once you have the variance, take the square root of it to find the standard deviation. Finally, subtract the mean from the data point you're examining, and divide the difference by the standard deviation. To learn how to calculate the mean of your sample, read on!. How to Calculate Z Scores. A Z score allows you to take any given sample within a set of data and to determine how many standard deviations above or below . The use of epidemiological tools in conflict-affected populations: open-access educational resources for policy-makers.