"Metaphor"? Umm, I'm not sure that's right. Rule of thumb, perhaps?
Hmm. Is there a convenient shorthand label for "something I can
evaluate but can't put on a pan balance"? How about _heuristic_?
One of my favorite rules of thumb is that "effort moves toward what
is measured--even (*especially*) when one is not conscious of the
measuring process". And of course, all measures are only estimates.
MMB, at but not for OCC