Two minus the mean and we take the absolute value. This calculation tells you how close to the mean your values are. When the number of classes is large or the value of f i and y i is large an approximate assumed mean is taken near the middle represented by A and deviation d i is taken into consideration.
It is called Assumed mean method.
In the above variance and standard deviation formula. X sample mean. The formula takes the absolute value of the difference between a value and the mean and divides it by the number of values. N Number of data points.