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UZR proponents would tell you that the system mostly captures park differences. Ibanez isn't playing in a uniquely small park, compared to all the -8 to -14 gloves in the league.
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But: assuming that you're correct, and a baseball park gives a player -23 in one year and another baseball park gives him +7 in the next ... well, that's my argument.
UZR isn't an absolute: it's warped radically by playing conditions.
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I agree with Blengino that UZR is a good starting point. What I argue is that it isn't the ending point for a dogmatic evaluation.