Monday, February 04, 2013
When do QB metrics stabilize?
Great stuff here! Love to see the techniques I use on baseball being applied to other sports. So, yards per attempt has an r=.50 after about 800 attempts. Basically, that means you need nearly two years for half the metric to be considered signal and half to be noise. He goes through some metrics, and determines:
| Stat | Formula | Stabilizes | Seasons |
| Sack% | Sack / Dropback | around 400 dropbacks | 0.75 |
| Comp% | Comp / Att | around 500 attempts | 1.00 |
| YPA | Yards / Att | around 800 attempts | 1.60 |
| YPC | Yards / Comp | around 650 completions | 2.15 |
| TD% | Pass TD / Att | around 2250 attempts | 4.50 |
| INT% | INT / Att | around 5000 attempts | 10.00 |
Note that because he took a QB's numbers in the same year, then something like TD rate does not necessarily reflect the QB's skill, but rather may have a bias in his receivers.
Interception rate being so low is also interesting. That may be because in order to survive, then you can't throw alot of interceptions to begin with. In order to find a signal in something, you need to have samples that have a large range in talent to begin with. If everyone has low interception rates, then it's harder to find out who is really really low in interception rates, and who is really low, and who is low.
This is why something like save percentage for goalies has low correlation: if you aren't saving pucks, you aren't going to be in the league long enough to be part of the sample. And this is why strikeout rates have high correlation: you CAN survive in the league if you have a very low and very high K rate. So, with a large range in talent, then you need less sample for the signal to get through.
Anyway, so that explains the sack rate stabilizing so fast: it's highly biased on the team's line, and staying away from sacks is not a primary requirement to being a QB (though it is a secondary one).
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