First of all, OBP is fueled by plate appearances and NOT at bats. As we know, walks aren't factored into at bat totals, so there's more than a 2% accuracy skew right there, right? Maybe Bill James will give you a real reason, but I've always understood AB's are used simply because that's the way the formula works best. Secondly, I'll never be convinced that a formula can ever be used to 98% correctly track a player's offensive contributions if it doesn't account for bad baserunning, smart situational hitting (i.e., not grounding out when there's 1 out and the tying run on 3rd) or at least SOME nod to hitting when men are on base. You can have your own beliefs, but I sinmply think most of that stuff simply balances out. Realistically, "bad" baserunning isn't a major factor over 162. Sometimes, you'll even be rewarded for bad baserunning, as the fielder will drop the ball, it'll short-hop the catcher and it will get past them, allowing a run and an advance, etc. Also, you concede that, on a player-vs-player basis, OPS falls below the 98% accuracy rate. How far would you say, though? I'd say it drops quite a bit, even when comparing a 2004 player vs. his own 2005 output. It's that tough to factor. I've never seen the team total at less than 90%, so I'd hate to go less than that. I'd say luck is a bigger factor than anything else not measured by OPS, the difference between a game ending DP and a game winning base hit is inches, and for the most part, isn't something the batter controls...other than how hard he hits it.