UZR Home & Away Splits

Ultimate Zone Ratings (UZR) home and away splits are now live in the splits sections for position players! And on the subject, I’ll add a quote from Mitchel Lichtman’s revised UZR primer, which we’ll be posting later today.

…if you still don’t trust a certain player’s UZR because of the park factors issue, you can check out his road numbers. Keep in mind that you will see lots of random differences between some players’ home and road numbers which have nothing to do with park effects – they are simply an artifact of small sample sizes. Remember also that even large sample sizes can have large random fluctuations as well.”




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David Appelman is the creator of FanGraphs.


11 Responses to “UZR Home & Away Splits”

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  1. Adam R. says:

    How about sorting tERA in the leader boards and team pages?

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  2. AK707 says:

    Wow, I just looked at Randy Winn’s UZR splits from 2008 and saw just how atrocious it is to play RF in AT&T park. I wonder if park adjustments can account for the ricochets of the archways?

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  3. MGL says:

    AK707, remember that all the UZR numbers you see on FG are park-adjusted (park-neutral) so, theoretically, you should not see any differences between a player’s road and home stats, due to park effects. Now, given the limitations of the park factors, as I mention in the UZR primer, that might NOT be the case. If you see a large difference between a player’s road and home stats in a “normal” park, that difference is likely just random fluctuation. If you see a large difference in a quirky section of a park, like RF in SF or LF in Boston, well, your guess is as good as mine as to how much of the difference is random and how much is due to park effects. Probably a little of both, mostly random. I would not put too much stock in the Randy Winn differences (I have not seen them), but if you want to put more weight in his (or any other player) road UZR, that is fine by me. Keep in mind that you are sacrificing sample size for some more accuracy.

    Also, it is not mentioned anywhere, but road and home UZR’s are zeroed out so that they will not reflect the fact that players typically play better defense at home than on the road (HFA). IOW, if a player were an average defender (0 UZR), he would likely be around -1 per 150 on the road and +1 at home, due to the normal defensive HFA. But, the way the home and road UZR are calculated, he is going to show up on FG as zero at home and zero on the road, since again, we are trying to capture context-neutral talent and performance (so basically we are adjusting for home and away).

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    • Colin Wyers says:

      I’ll admit I’ve missed some part-time (below 100 innings in that season) players, but I’ve compiled the UZR home/road splits for Mariners outfielders from ’02-’09.

      Home: 126.3
      Away: 44.6

      That’s a startling difference – about 80 runs – for a stat that, as you point out, has been adjusted for park and home field advantage. And there’s nearly 2000 defensive games in each half of the split, so it strikes me as unlikely that random variance accounts for it, either.

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    • B says:

      When you say it’s zeroed out, how is that adjustment made? Do you just apply a league average home v road adjustment to every player?

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  4. tangotiger says:

    What I would like to see is Team splits like this:

    TeamX – At Home
    TeamX – On Road
    Opponent – At TeamX’s Home
    Opponent – At Opponent Home of TeamX’s games

    Only in this way can we see that the park splits make sense.

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  5. MGL says:

    Colin, interesting. Here is the deal though:

    Let’s say that you were compiling park factors for a certain park, say Wenfay Park. Could be for pitching, batting, or defense (UZR) – it doesn’t matter. And let’s say that the ratio for 3 years of home plus home opponent to road plus road opponent (the usual way we do park factors) was 1.50. So the unregressed park factor for 3 years of data would be 1.50.

    If we used 1.50, the unregressed PF, to adjust players in that park for those 3 years, we would find that, by definition, those players would have pretty much the same park adjusted stats at home and on the road. I say pretty much because the 1.50 includes not only the home players home and road stats, but their opponents as well. So everything looks good, right? Park adjusted home and road stats are equal, as they should be. The problem with that is that park adjustments should NOT make home and road stats equal!

    Using 1.50 is a terrible PF! Certainly that has to be regressed, assuming we don’t know much about the park. Depending upon what kind of park factor it was, that 1.50 would probably get regressed to at least 1.25 for only 3 years of data. But, if we use the more correct PF, 1.25 (as an estimate of our true PF), the home players will NOT have the same park adjusted home and road stats! So which one is correct?

    The second one of course. Basically, the home and road stats should NOTbe the same for in-sample data (the sample of data that is being used to generate the park factors). They should NEVER be the same unless the PF was 1.00.

    Now, if you look at 2010 and beyond (data was not used to make the the PF) and we still continue to get the kind of park adjusted splits that you are getting, then we may have a problem.

    But, for the 2002-209 data, OF COURSE the data is going to look like that! That is the data that the PF’s were based on and the PF’s were regressed to park adjust the stats!

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  6. John says:

    How do I find opponents ERA at Coors Field by year. Thanks John

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