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Monday, November 16, 2009
The ten year aging curve
Suppose I get all players born since 1895 who had at least 300 PA from the ages of 27 to 36 (they may have played before 27, they may have played after 36), and we only focus on their stats at those ages. I count 179 players. Here are their average LWTS per 700 PA, at each age:
Age avgLWTS_700
27 20.2
28 22.8
29 22.3
30 21.7
31 24.6
32 22.6
33 19.0
34 18.8
35 13.4
36 11.6
The peak is at age 31.
What if I take a 10 year period from age 22 to 31? I’ve got 116 players:
Age avgLWTS_700
22 10.7
23 18.3
24 21.2
25 24.5
26 24.6
27 24.6
28 23.6
29 24.3
30 20.6
31 20.8
The peak is at age 25-27. Age 31 doesn’t look so good now.
What if I take age 31-40? Only 42 players:
Age avgLWTS_700
31 31.3
32 30.4
33 26.8
34 28.6
35 25.3
36 25.0
37 26.3
38 22.1
39 16.9
40 12.7
Peak is at age 31. (D’uh) And age 34 is not that far off. Why is that? Because we pre-selected our group to have 300 PA every year through age 40. OF COURSE they are going to be really good hitters at age 34, and even age 37 (just 5 runs off their peak).
Clearly, deciding what 10 years to include in your time frame will bias your peak period.
So, when I see JC say:
According to my estimates, a hitter who has a .900 OPS at his peak would be expected to post around an .850 OPS at 35; a pitcher with a peak 3.5 ERA is expected to post around a 3.75 ERA at 35. Yes, age saps athletic skill, but the stock of skill being diminished is also important.
I’m there with my jaw open. Are you kidding me? A pitcher, who peaks at age 29 or 30, will, at age 35, have an ERA that is just 0.25 worse? How is this possible? Well, if you knew, at age 29 or 30, that he would STILL be pitching at age 35, and that in-between he met some minimum IP threshhold every year, then yes, sure, his ERA might be just 0.25 worse. That’s just like my examples above. Ray Fair, in an economic paper I panned, made a similar incredulous claim:
His new paper on page 13 says: If a pitcher’s peak ERA is 3.50 (the mean of ERA in the sample is 3.50), then the 0.314 value for R37 means that his predicted ERA at age 37 is 3.814, an increase of 9.0 percent.
That is, he has the same conclusion, which is absurd on its face. Who are the 37 year old pitchers in his study? It surely is a subset of the 27 year old pitchers. And, that subset is all the good pitchers. What a terrible choice he made here.
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