Important: The stats package has been deprecated. Use the superseding package Statistics instead.
The data 1,3,7 is more dispersed than the data 2,3,5, as can be seen as follows:
Here is an example where the mean deviation is not a suitable measure of dispersion. The data consists of 100 copies of the value 1, and 1 copy of the value 100000. This last value really has too much of an influence (it is likely that it was a bad reading of an instrument).