You can have a calculated field that performs a table calculation:
Latest Score = LOOKUP(max([TABE Last Math Score]),last() )
Previous Score = LOOKUP(max([TABE Last Math Score]),last() -1)
Test Gain = [Latest Score]-[Previous Score]
The key is editing the table calculation once you place it in the view.
I updated your workbook with these formulas. Sheet 2 shows how you can use the test gain in a different visualization.
By they way, since you have duplicate records by client/date, I used Max(Score) instead of Sum(Score).
Is this what you were looking to do?
TABE Scores for Forum.twbx.zip 42.2 KB
This is great! Thanks so much Pedro.
I've got a similar situation but a bit more complicated. But still attempting to calculate gains. Can you help?