Perhaps simplest to understand with some sample info.

Doctor begins office visit with a time-stamp 8am on a record. Store to Var1
Read many more also storing to Var1
Doctor makes last entry at 8:15 also storing to Var1

At the group level, I can determine the min as 8:00 and max as 8:15
Therefore at group I can know time elapsed was 0:15

Do this for all the office visits, creating printout like
Doc1 Pt1  8:00 8:15 0:15
Doc1 Pt2  8:20 8:25 0:05
Doc1 Pt3  8:25 8:55 0:30

Because I have already done some 'math' on the data, that seems to prevent me from doing math on the group level math
So, do I try one of these or something else?
(a) Create some running-total variables to know Doc1 saw 3 pts for 50 minutes (and 16.7 minute average).
(b) Save this file to Excel and then write a 2nd CR to ODBC connect and read all the lines in this 2nd report create the summary/table.