Look at Average – Which average?
Concerning your expressions:
1. You shouldn't have naked field references (See Use Aggregation Functions!):
2. You need to wrap your Aggr() function in an aggregation function. Otherwise you will have exactly the same problem with the Aggr() as you have with naked field references: If there are multiple values, it will be interpreted as NULL. Hence:
Avg( Aggr( ... ) )