I have been studying statistics hard these days, and when I read the content of "describing the trend of data dispersion", I talked about the MD of mean difference.

The book says that mathematicians hate and despise absolute values, and then they introduce the method of taking square as positive, introducing variance and standard deviation.

"Like the mode, median and range, absolute value and mean difference (MD) can't be algebraic, so I won't go into details here."