If you're trying to name the least reliable statistic in professional football, there's a healthy field of candidates. A punter's average distance is basically irrelevant, any measure of yards gained is always a "give or take a few" proposition, and please don't get anyone started on "quarterback rating."

Still, if you ask players and league insiders, the most maddeningly imprecise football statistic is something that seems simple enough: the tackle.

In...