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I'm not saying the NL is filled with crappy players and a few starts or that the AL is ten times stronger or whatever...but there's enough of a difference for fringe players to be terrible in the AL and good in the NL (or at least to APPEAR that way).
And it may be correct that the NL is 80% of the AL with 20% crap thrown in on top that screws up the statistical picture...that dosn't change the reality that you need to measure league quality and make a strong guess as to how it will impact each player making the transition