The Mean Absolute Deviation of a data set is the average of the sum of absolute deviations.
Example
| Quarter | History | Forecast |
|---|---|---|
| 2010-Q1 | 22 | 25 |
| 2010-Q2 | 19 | 21 |
| 2010-Q3 | 20 | 22 |
The sum of the absolute deviations is: |22-24| + |19-21| + |20-22| or 6.
The mean is 6/3 or 2.