Question

The difference in a variable measured over observations (time, customers, items, etc.) is known as

Answer 1

The difference in a variable measured over observations (time, customers, items, etc.) is known as the variance. 
it is the measure of variability that utilizes all the data and it is calculated by
 taking the differences between each number and the mean,. Then these differences are squared in order to be positive. At the end the sum of the squares is divided by the number of values in the set.