Thank you for your well-thought comment. One of the desiderata used to derive the original product rule is to use real numbers to represent the degrees of plausibility. So, it will be very interesting to see if the result still holds if we relax it to be a complex numbers.
I've taken the test not long ago and I agree it is quite informative. One concern I always have with these kinds of tests is that we tend to select options (at least in some cases) that highlight who we want to be rather than who we really are. I would like to know how this kind of bias is reduced.
It can be any real-valued measurement of objects, as long as we can reasonably assume the three assumptions are satisfied.