write a script, and go for the loop, checking for no. of occurences of that serial no. in table (Testing). and store it(i.e. no. of occurences) in ur field of table Priority. and do this looping for each record of the table(Priority).
Thanks for your reply, I think perhaps I didn't explain what I need very clearly.
Each serial number in 'Testing' has many tests with one entry per test. The test score is an integer between 1 and 100 and is stored in the field 'Testing::Test result'. I want to find the highest test result for each serial number in 'Testing' and store it in a different table.
I think your script idea will count the number of tests for each serial number, but doesn't find the highest score.
Max ( RelatedTable::Score )
Will return the highest score of the records with a matching serial number.
Thanks very much, that's exactly what I was looking for. I'm ashamed I hadn't already discovered the Calculations dialog!