I don't claim to know the reason for it, but I do think it's worth mentioning that this improvement is sustained over a pretty long period of time (this is the third year of a batting average well in excess of .310, and the 5th consecutive year of improvement in batting average). It's also not entirely unprecedented. Check out Barry Bonds (are we allowed to even refer to him anymore?)... His batting averages were pretty consistently in the .290 - .310 range and then from 2001 - 2004 (ages 36-39) he pulled off a pretty nice streak of 328, 370, 341, 362). If that example is too pungent for you, how about Tony Gwynn, who from 1993 - 1997 (ages 33 - 37) ripped off averages of 358. 394, 368, 353 and 372, when his career batting average entering the 1993 season was "only" 327.

Paul Molitor from ages 34-39 is another great example. During that stretch he batted .322 and the rest of his career was just .299

Bottom line, older players are constantly making adjustments and trying to improve, just like younger players. They have an added "drag" on their ability to improve - the fact that their bodies are slowing down, but this drag is not something that cannot be overcome for a sustained period (even if eventually they are assured of slowing down).

Aryeh

Chipper isn't having a tremendous jump in performance. He is hitting singles, not long balls. Think Gwynn rather than Bonds.

Chipper's unique in a way. This is how I see his career BA...

When he was adjusting to the majors (not an accurate gauge of his talent)...
thru 1995 : .268 (.355 OBP)

Adjusted to MLB life...
1996-2001 : .313 (.407 OBP)

When he was learning to play LF (not an accurate gauge again)....
2002-2004 : .295 (.404 OBP)

After his position and role on the team settled....
2005-today: .336 (.429 OBP)

There's two periods there that can be reasonably dismissed when trying to figure out how good a hitter Chipper really is. That isn't true with most players, but it seems to be clear to me with him.

His true potential level at the plate wasn't really there when he was coming into the league or when he was being asked to make a huge shift in learning a new defensive position. But when he's just being Chipper... he hit .313 and even .336 after returning to his comfort zone.

Even if one says I'm silly about all this... he's been batting .316 since the start of '98. So, over the past decade he's been more than a .310 hitter even when you include the drag-down-Chipper's-BA years of '02-'04.

If one does combine his best periods ('98-'01 & '05-now), he turns out to be a whopping .325

And still, his BA when you simply toss out his distracted slump years is .320 ...which I believe is probably the most accurate measure of his batting ability.

Which still means he only needs to hit 80 points higher for the year to reach .400...which doesn't really seem that impossible. I suspect he's altered his swing and possibly isn't swinging for the fences as much. I wish I could find good video of him swinging 8 years ago and swinging today.

David,

If you check my blog, and look at the link from VegasWatch, you will see that our estimate of Chipper is now around a .330 hitter.

You have to remember that coming into this season, whil Marcel had him as an estimated true .310 hitter, that was simply an estimate as of Apr 1, 2008. But, we now have more data that we need to parse. And that extra data now gives us a revised estimate of around .330.

I urge you to update your graph to reflect that.

Tom

Gwynn and Molitors increases came with a jump in league offense. I understand those. Paul O'Niell had the same thing happen. Chipper is seeing this surge in a year where offense is down.

Dave:

I guess I would throw the question back to you and ask you what it is about 24-27 year olds that makes it so understandable to you why they would exhibit improvements in performance and what it is about Chipper's age that PRECLUDES such similar improvements in performance. Less likely from a 34 -36 year old? Sure. Impossible? Not sure why that would be so. Can you enlighten us?

Thanks,
Aryeh

FYI - Interesting article by Nate Silver on Chipper's current "true" level of ability (Silver puts it at .348)

http://www.baseballprospectus.com/article.php?articleid=7652

Aryeh, because players peak at age 27. If some makes improvement during a time when you expect them to improve, that's not a surprise. If someone makes a big improvement when most players his age are out of the game, then it's a puzzle.

The Wall St. Journal's "Numbers Guy", Carl Bialik, has a column on this topic today -- he cites his discussion with David to explain to non-baseball people why it's sensible to use the lifetime average rather than the current one to estimate hit probability.

You keep mentioning how he hasn't had a chance to "wear down" yet, both in this season and recent ones, but he has career .305 pre-ASB BA, and .317 afterwards.

On the whole players tend to peak at 27. But it's not true for every player.

I still am unclear as to what you attribute the increase in performance from 25 to 26 and from 26 to 27 and why that couldn't also apply to someone 34 years old.

David,

For Gwynn (and Molitor) the league BA increased by about 10 points. Gwynn's BA increased about 50 points at ages where he should have been declining. I think these are reasonably good examples to compare to Chipper.

Also the BA in the NL this year isn't too different from what it has been for a while (it's .259). It's the AL that's seeing a drop off and that's mostly in SLG.

Did he have laser surgery? That could make a difference. Here are two articles that mention the effects:
http://www.nysun.com/sports/day-delgado-night-delgado-are-different-hitters/76612/
http://www.hardballtimes.com/main/content/email/new-eyes-better-bats/