If x_{1},x_{2},………………,x_{n} be the n observation then M.D is defined as

Where A may be mean (.X), or median or mode.

For simple frequency distri0ution, M.D. is defined as

where A may be mean (.X) or median or mode.

For grouped frequency distribution, *x *will be taken as class-mark.

Sometimes M.D. about mean (.X) is called simply M.D.

**Note. **1. M.D. taken about median is the least, compared to M.D. taken about mean (.X) or mode.

2. It is not a very accurate measure of dispersion particularly when it is calculated from mode.

3. S.D.> M.D.