UZR sometimes has wide variations in a given player's rating , even though the player's range does not vary very much. I have no faith in it. Range Factor is a Bill James stat that relies mostly on putouts and assists.
RF/9 is basically what you said we should do: count POs and As and figure in the innings played. It is PO+A divided by innings x 9, so you get a number of how many plays they make per 9 innings. Of course, players who play on teams with pitchers who strike out a lot of players will get less chances, high GB pitching teams will hurt OF'er numbers, and high Fly Ball pitching sstaffs will hurt IF numbers, but RF/9 is clearly better than just looking at POs, Assts and innings and trying to compare 3 stats at once in your head. It does it for you.
It is hard to put a number on how many errors are acceptable as compared to outstanding plays.
It's easy for me. I play made that other players don't normally make is about equal to an error taken away from his total, unless it's an error that allows a runner to advance to 2B or take an extra base. Clearly, A SS who makes 50 more plays than another SS, but makes 25 more errors is the better SS. There's no way 25 more errors is not more than made up for by creating 50 more outs than the other guy. There are SSs in MLb who routinely, year-after-year, make 50-100 more plays in the field than specific other poor ranged SSs. That's a huge differential that is not measured by Flg%. RF/9 might cover some of it, but fo rthose who look at flg% first, it is often lost.
I watch a lot of games , and I prefer to use my own opinion as to a player's defense. If you know the game , you know a great defender when you see one. ( e.g. Iglesias ) You also know a butcher when you see one.
I do too. I watch every single play of every Sox game at least once. I also watch a lot of other baseball. I have done this for years. I played the game and always loved defense. I'm not saying I am a better evaluater than you or anyone else (except softy), but I have always concentrated on SS defense more than any other area of the game. I am certain that a great ranged SS can save over 100 hits a season over a below average SS. That is huge!
Let's take one example: Julio Lugo. I was highly critical of this guy as our SS and about his not meeting offensive expectations either, however, when he was healthy, he had very good to excellent range. True, he botched some easy plays, and in his later years with Boston, due to injuries, his range declined sharply to the point that he was one of the worst SSs we have had playing FT.
In 2003 he had a RF/9 of 4.75
Begins career with Boston....
2007 4.21 (still decent)
2009 4.04 (some in STL)
Most remember his last year and half here in Boston, but IMO, we don't win a ring in 2007, if Lugo had a 3.70 RF/9 at SS (his 2008 number).
Frankly , I don't even try to figure out how they compute UZR and WAR. Sometimes we can get too convoluted with all these numbers.
UZR is formulated by people who watch every play of the game. They judge balls that are put in play as makeable or not. It is not a perfect science, but do any of us watch every game and play of every player? It's impossible. Yes, we watch the opposing team's SS and wish ours was as good as theirs more often than not, but that is not really a true comparative analysis. Although there are flaws in the UZR system, especially over shorter sample sizes, these guys are probably better experts than I am, and try to be objective, unlike the "official" Yankee scorer who lets Jeter off the hook so many times a year, it's not funny. (All home scorers due to some degree.)
UZR is part of WAR. WAR is not perfect either, but again, look at the top 10 players by WAR over the last 3 years and comapre them to the top 10 OPS or BA, or HR or OBP, or whatever stat you choose, and as a single number, the WAR number ranks the best players better than OPS (the common offensive stat of choice these days) alone, since it factors in defense.
Top 10 by WAR from 2010-2012:
10) C Gon
BA: Miggy, Votto, Braun, posey, VMart, Mauer, Beltre, Hamilt, CGon, AGon
OBP; Votto, Miggy, Prince, Baut, Mauer, Papi, Berkman, C Ruiz, Holliday, Braun.
SLG: Miggy, Baut, Hamilton, Votto, Braun, beltre, Papi, Stanton, Pujols, CGon