I used a certesian join to find a single average (called ClassAverage) of another average (StudentAverage) over multiple records. However, if one of the records doesn't have a StudentAverage (if one student hadn't taken a required test) then the ClassAverage won't compute. Is there a way to limit the ClassAverage, either in field definition or in the relationship definition, so that the ClassAverage only uses data that fills certain criteria (IsEmpty or something else that I add later)?

You didn't specify if you wanted to include or exclude a test result of 0 (not taken) or not. That answer will define your appoach:

Exclude Empty Results

You're apparently using FM7, so life will be a little easier for you. You have the ability to apply multiple criteria when you create your relationships, so just add a rule to accept positive values.

Include Empty Results

Be certain the 'Do not evaluate if all fields are empty' is unchecked, and additionally it would be wise (in this case) for auto-enter and calculations to set fields so zero rather than nulls.

Thanx for the help. However, one more question, is there a way to limit the relationship even more? Say for students who are taking the class in a specific term (defined as TermCode)?