There's a big difference between subjective and voodoo. By that definition an umpires strike zone is voodoo, I think all non cynical baseball fans would agree that while umpires can have a bad day and be a little off a strike zone is subjective. It might vary but it's pretty darn close, and like Moon said they have trained observers.
Even though we know umpires are imperfect we trust them enough to never blame the umpire for a pitcher with bad control. No one would ever say that we can't trust Henry Owens WHIP, and BB/9 because umpires strike zones are voodoo....you just wouldn't. Defensive metrics are very similar in this sense, people just don't see it that way because it's new and what is new scares people.
Yes, very well said.
The payed and trained observers who analyze each play in person are trained and rotated, unlike hometown scorekeepers.
Sure, there's some subjectivity and imperfections involved but the idea is rather simple.
Most fans get to see their own player every game or nearly every game. They see other team's players on defense rarely, some very rarely. If we did get a chance to see every play made and not made by every player over a season, we'd have a better understanding of how our players are on defense- good or bad. (That's assuming we can stay objective.)
The observers with UZR/150 are probably more objective than us, and you'd think with all the observations they make, they'd be better than we are at determining what play was makeable or not.
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I don't totally trust their numbers, and even they say we should only look at large sample sizes (sometimes larger than a full season) to make any definitive judgments on a player's defensive ability.
UZR/150 is not perfect, but it beats Flg% and RF/9. It beats the two combined. It takes into account how many balls are hit to a player, how many were easy, medium hard and in-between. It beats my own personal observations by themselves, because I don't know jack about how good the Colorado Rockies SS is defensively through observations and looking at Fldg% and RF/9 only. If I don't know how good he is, how can I rank Bogey comparatively?