and thanks for your help! Without helper columns/rows I'm trying to find the average difference in LVEDi between first and last visits of the patients.

E.g., by hand you would work it out as:

```
(diff for patient a + diff for pt b + diff for pt c)/3 = 
((55-45) + (40-31) + (25-31))/3 = 
(10 + 9 + (-6))/3 =
13/3 =
4.333

```

I'm sure the answer involves indexing with an array function, but I can't get started on how to tackle it!

Any help?

Thanks!

```
      A          | B      | C          | D      | E
 01   Patient ID | Date   | First-Last | LVEDi  | LOTS of similar cols
 02   a          | 3/3/03 | LAST       | 55
 03   a          | 1/1/01 | FIRST      | 45
 04   a          | 2/2/02 |            | 65
 05   b          |        |            | 
 06   b          | 3/2/16 | LAST       | 40
 07   b          |        |            | 
 08   b          |        |            | 
 09   b          | 2/1/12 | FIRST      | 31
 10   c          | 1/7/14 | LAST       | 25
 11   c          |        |            | 
 12   c          | 2/2/03 | FIRST      | 31
 13   c          | 2/2/08 |            | 46

```