Sunday, June 01, 2008

Why are runs so scarce in the 2008 American League?

Last month, J.C. Bradbury showed that comparing April home run rates to average April temperatures showed a surprising correlation: the two curves moved together almost in unison.

So is this year's drop in home run power due to the weather? Apparently not. In a
follow-up post, Bradbury shows that if you look more closely at the relationship between game-time temperature and home runs (outdoor games only), only 4% of this year's decline can be explained by the cold.

Also, and more interestingly, home runs are down a lot more in the American League than in the National League. In fact, AL offense is actually lower than NL offense so far this year: at time of writing,
4.38 runs per game vs. 4.60 runs per game. That's a big difference, especially considering the AL, with its DH rule, is normally about half a run *higher* than the NL. So the American League is about 0.72 runs below where it "should" be, although you have to adjust for interleague games, in which the DH advantage disappears. Call it, say, 0.6 runs per game.

Could it be that AL cities have been colder than NL cities so far this year? Nope. Bradbury
checked that too, and it turns out that the temperature in AL parks has been pretty typical this year.

And to put one more nail in the temperature coffin,
Zubin Jelveh looked at domed stadiums, and found the same sharp dropoff in slugging percentage as in other parks.

So what's causing the drop? One theory is that it's not a dropoff in hitting, but, rather, an improvement in pitching. Supporting that theory is that, in interleague play to date, the AL is three games above the NL, suggesting that it's still the better league. Of course, that could just be random chance, because there haven't been that many interleague games so far this year. (I couldn't find exact interleague records, but the AL as a whole has three more wins than losses, and the NL vice-versa.)

Another theory is that it's the PED clampdown causing the drop in power, but, as others have pointed out, steroids testing actually started several years ago, and, in any case, it doesn't make sense that users would be so heavily concentrated in the American League.

Over at the Sporting News, David Pinto suggests that it's an age thing. He notes that AL hitters are significantly older than their NL counterparts, 29.5 to 28.8. That's come as a reversal from 2005, when then AL was actually 0.4 years *younger* -- a four-year change of 1.1 years.

But there's no hard evidence that younger players are actually better than older players – looking at Pinto's (very interesting) charts, you note that the NL has better younger players, and the AL has better older players. But the AL old-guy advantage is smaller than the NL young-guy advantage. As Pinto writes,

"... at the ages where the NL OPS is higher, it tends to be much higher than it is at the corresponding AL age. Where the AL OPS is higher, the gap is not quite as large."

So it seems to me that it's not as simple as an age thing.

It's an interesting problem, and I don't know what the answer is. But any theory would have to explain:

-- why the sudden drop
-- why the drop is so much larger in the AL
-- and why the AL is ahead in interleague play, despite its anemic offense.



At Tuesday, June 03, 2008 10:26:00 AM, Blogger Don Coffin said...

It's early, it's random?

At Tuesday, June 03, 2008 10:56:00 AM, Blogger Phil Birnbaum said...

Hmmm, let me check ...

I think the variance of runs per game is 9. Assuming 50 games per team so far, and 15 teams each league ... that's 750 games. That's an RPG variance of .11 runs squared per league, or an SD of .16 runs for the *difference* between the leagues.

Assuming the discrepancy is .72 runs, that's well over 4 SD.

At Tuesday, June 03, 2008 12:21:00 PM, Blogger Unknown said...

The OPS for this April was the exact same as the OPS for last April. All the difference came since May.

At Tuesday, June 03, 2008 12:58:00 PM, Blogger Tangotiger said...

Phil, I talked about this on my blog a couple of weeks ago (Joe Sheehan article). You need to do this by per 27 outs, not per game, as the AL games were shorter this year. Also, you are better off just focusing on OBP and SLG, and comparing that year-to-year.

And the idea that you need more runs in the AL: that's only true if "all other things equal". Parks is the great unqualizer. And of course, if there's been a shift in the balance of pitching/hitting talent in each league.

At Tuesday, June 03, 2008 1:18:00 PM, Blogger Phil Birnbaum said...


Well, that's part of the answer then, that games in the NL were longer than games in the AL. That's a good catch, never thought of that.

Oh, here's a link to your blog post.

And, agreed that parks and talent could be causing the effect ... but the parks are the same as last year, and there's normally not THAT much player movement over one off-season (is there?). The NL having younger players, though, is probably responsible for a shift in talent, because young players tend to improve and older players tend to decline.

So we have:

--NL is playing longer games this year
--NL is younger
--Talent shifts over the off-season
--Park changes over the off-season

My gut feeling: 65% luck (including luck caused by imperfectly converting OPS into runs), 15% longer games, 10% talent shifts because of age, 5% talent shifts off bodies moving, 5% random talent shifts (injuries, etc.), 0% parks.

I am open to other opinions -- as I said, this is my gut feeling, and I'm even too lazy to convert the OPSs into runs. Tango, your gut is probably better than mine on this, what does your gut think?

At Tuesday, June 03, 2008 9:55:00 PM, Anonymous Anonymous said...

Could older players be worse fielders?

Probably not much, but just one idea

At Thursday, June 05, 2008 11:14:00 AM, Blogger Tangotiger said...

"Parks" includes everything about it. Not only the location, but the dimensions, the field, the temperature, the rain, the humidity, the daylight.

When I looked at it last month, I saw nothing to worry about. I'll have to look again.

At Thursday, June 05, 2008 11:21:00 AM, Blogger Phil Birnbaum said...

Sure, if "parks" includes weather and day/night, agreed that there might be something there. I should have included those somewhere, and parks is as good a place as any.


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