Range of Activity DurationI covered a lot of related material under Standard Deviation and Activity Variance already. Once you have calculated the standard deviation of an activity, and the activity variance the range of activity duration is simple. The formula is just EAD +/- SD. So once you have done the more involved calculation of the EAD and the SD here you are only calculating their relationship.
But what does that mean? In this case the standard deviation is an estimated fudge factor on your predicted duations. Adding and subtracting the Standard deviation just provides a range of variance. If this calculation looks similar to the Rough Order of Magnatude (ROM) calculation that's because it is. The key difference here is that ROM uses large rough numbers in lieu of reliable estimates and hard metrics.
For Example: Assume we intend to produce 5,000 widgets as a project and belive that will take 90 days. We further calculated that this estimate could be up to 15 days off in either direction by calculating the activity variance. Therefore:
90 + 15 = 105
90 - 15 = 75
105 - 75 = 30
So here we can say that the Range of Activity Durations = 30