Subtract the mean from each of the data values and list the differences. It is a statistics found from a collection the sample of data on one or more random variables. This article tells you how to find the sample mean by hand this is also one of the AP Statistics formulas.
It is otherwise called as empirical mean.
Divide sum by the number of samples. You estimate the population mean and standard deviation from a sample or samples. Add up the sample items. Square each of the differences from the previous step and make a list of the squares.