You can add the following column in a project report, to show the Actual Duration aggregated as an average in a grouping

To add this column to a project view:

1. (Recommended) For best results and to see the aggregated average value of the Actual Duration, you must have a Grouping added to your project list or report. 
   For more information about creating Groupings, see "Creating and Customizing Groupings."
2. Navigate to an existing project view.
3. Expand the View drop-down menu, and select Customize View.
4. Click Add Column.
5. Click Switch to Text Mode.
6. Mouse over the Show in this column area, and click Click to edit text.
7. Remove all text in the Text Mode box, and replace it with the following code:
   

   aggregator.displayformat=compound 
   aggregator.function=AVG 
   aggregator.namekey=view.relatedcolumn 
   aggregator.namekeyargkey=actualduration 
   aggregator.valuefield=actualDurationMinutes 
   aggregator.valueformat=val 
   displayname=Project Actual Duration 
   durationunitfield=durationUnit.value 
   linkedname=project 
   namekey=actualduration 
   namekeyargkey=actualduration 
   querysort=actualDurationMinutes 
   textmode=true 
   valuefield=actualDurationMinutes 
   valueformat=compound#M:D 
   viewalias=actualduration

8. Click Save View.