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Tuesday, November 22, 2011
Selective end points AND data mining AND publishing bias rearing their ugly heads again…
Let’s see how many posts it takes for the geniuses on BBTF to figure this one out. So far 9 and counting…
Anyway, here is the link:
http://www.thegoodphight.com/2011/11/21/2485197/phillies-citizens-bank-park-not-a-hitters-haven
to an article which tells us that CBP has played almost neutral for the last 4 years, therefore it is now a neutral park, as opposed to the first 4 years when it was an extreme hitters’ park (around 1.07).
Let’s forget for a second how a park can all of a sudden change its true PF’s (it can’t other than by changing other PF’s in the league and even then it won’t change much - of course the “effective” PF can change - a little - with weather and with different players).
Instead, let’s do this thought exercise:
You have 30 parks with a true PF of x, y, x, etc. I am telling you that they never change (which is actually reasonably true, as I indicated above, barring a remodel of course). We track the observed (sample) PF’s for 8 years. What are the chances that in the last say, 3, 4 or 5 years (you get to choose the end points) some park will show an observed PF that is quite different than its true PF AND/OR quite different than the observed PF in its first 3, 4, or 5 years?
IOW, what can we conclude about the true PF of CBP? Not much other than its true PF is likely the un-weighted average of the observed PF over the last 8 years, regressed toward some mean (of a similar park, dimension, weather, altitude-wise, etc.). If you want to weight more recent years slightly more than past years, I don’t have much of a problem with that, although I don’t think that any weighting is necessarily appropriate…
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