Though sometimes it's even more useful to acknowledge that the sharp-boundaried concept we're testing for is different from, though perhaps expected to be correlated with in some way, the fuzzy concept we were initially interested in.

That helps us avoid the trap of believing that 17-year-olds aren't responsible adults but 18-year-olds are, or that 550-word essays are long and rambling but 450-word essays aren't, or that food is safe to eat on September 25 but not on September 29. None of that is true, but that's OK; we aren't actually testing for whether voters are responsible adults, essays are long and rambling, or food is expired.