But then how do you come up with a rational set of metrics? It's possible in baseball as baseball can be reduced down to a battle between pitcher and batter with infielders and outfielders paling in significance. On the other hand, football is a sport of many moving parts. The statistical output of a QB is a function of their own performance but it is also a function of their OL's performance, their supporting cast's performance, the opposing DL's performance, the opposing LB's performance, the opposing secondary's performance, the play-calling and emphasis of the offense, the play-calling and emphasis of the defense, the weather, situational circumstances etc. Taking that altogether, it becomes extraordinarily difficult to disentangle QB output from QB performance, something which makes constructing a rational metric nigh-impossible.