Question

Who has created a distinction between common and special cases of variation?

Answer 1

Every piece of data which is measured will show some degree of variation: no matter how much we try, we could never attain identical results for two different situations—each result will be different, even if the difference is slight. Variation may be defined as “the numerical value used to indicate how widely individuals in a group vary.” 

In other words, variance gives us an idea of how data is distributed about an expected value or the mean. If you attain a variance of zero, it indicates that your results are identical—an uncommon condition. A high variance shows that the data points are spread out from each other—and the mean, while a smaller variation indicates that the data points are closer to the mean. Variance is always nonnegative.