You've received great info from aeh, as usual.
To clarify, when comparing scores between various test instruments, a collection of paired scores from each of the test instruments is assembled and analyzed. Each test-taker will have taken both of the tests, and submitted the documentation of their test scores.
This Hoagies' webpage describes the process:
Data Collection: Old and New IQ Test Score PairsIt may a bit like contriving an equivalence table between ACT scores and SAT scores... each of which measure different things.
