Nico at Athletics Nation has some ideas about whether the A's, if they add a big bat, should add a lefty to mash righties or a righty to mash lefties. Here's his post in an iframe. I'm not sure what this is going to do for those of you on the feed, but I'll find out in a few hours when I get the feed in my inbox.

He notes that despite perception, the A's hit .271/.346 (AVG/OBP) against lefties this year but just .259/.324 against righties. My first idea was to check slugging. Why'd he leave it out? Turns out the A's slugged the same against the two sides: .407 against lefties, .408 against righties. That means that the ISO, a better measure of pure slugging, was about ten points higher against righties than lefties (because of the lower batting average), but ten points isn't huge by any means.

On to this other ideas, then. First, that perhaps the A's just faced a higher proportion of lefties while they were "hot" than they did while they were "cold." At first glance, of course, before the hypothesis even gets tested, we have the question of causation. Did the A's get hot because they faced lefties, or did the A's happen to face lefties while they were hot?

Eyeballing Baseball Reference's awesome team schedule breakdown (which has a visual representation of the team's streaks - go check it out to see what I mean), we can break the season down into a few chunks. Through May 1, the A's were 13-12 and never had a streak (winning or losing) longer than three games. From May 2 through May 29, the A's went 4-20, so that's a notably bad stretch. Then from May 30 through August 30, Oakland went 58-24, which is pretty good baseball. From then to the end of the season, they went 13-18, another mediocre-to-bad stretch.

So let's see what kinds of pitchers they faced during those periods. I'll start off just worried about starters for two reasons: 1) It's easier; 2) I'd guess that bullpen usage isn't going to vary that much because of managers playing the matchups. (Note that this is an untested assumption; I just don't have as much time as I'd like to go into it more.)

From the start of the season through May 1, the A's faced 20 righties and 5 lefties. It's nice that it broke down so evenly: that's 80% righties, and the A's won 52% of their games.

From May 2 through May 29, the A's faced 16 righties and 8 lefties. That also breaks down nicely. The A's won just 17% of their games while facing 67% right-handed pitching.

From May 30 through August 30, the A's faced 55 righties and 27 lefties. That's very close to nicely, but not quite. The A's won 71% of their games while facing 67% right-handed pitching.

Finally, from August 31 to the end of the season, the A's faced 25 righties and 6 lefties. That works out to 81% righties and 42% wins.

Is there a pattern? When the A's were mediocre, they faced a lot of right-handed starters. When they were awful, they faced a lot of lefties. When they were awesome, they faced a lot of lefties. We could say that the A's play to extremes against lefties, but that seems unlikely.

Next idea? That a couple of A's hit lefties better than they should have been expected to. ESPN.com gives you split statistics in a heartbeat, so that makes our job relatively easy here. The guys who hit lefties well are Mark Ellis (313/403/509), Bobby Kielty (322/398/469), Bobby Crosby (314/391/549), Eric Byrnes (329/389/610), Erubiel Durazo (350/381/600), Mark Kotsay (324/359/458), and, for on-base ability, but no real power, Dan Johnson (283/395/404) and Jason Kendall (293/366/335).

Now, Durazo only got 40 at-bats against lefties before going down with his elbow, so we'll throw him out. Of course, all the samples are small, but the next-smallest number of at-bats is Eric Byrnes's 82, which is not so out-of-line with Ellis (112) and Crosby (102) that I want to throw him out.

Ellis hit 258/333/413 in the three years (though for him, he only had two years) before 2005, so he hit significantly better against lefties this year. That said, his overall performance was so good that I don't think his performance against lefties was fluky. His isolated on-base percentage is about the same as it was before, but his isolated power is greater, both in his split and in his overall numbers, and that's something we might expect as a player gets older, especially one who was small and might have filled out at this point. In other words, while his performance was a pleasant surprise, I don't know if the A's should worry about it dropping off next year to the point where the team's numbers against lefties will decrease.

Bobby Kielty was brought to the A's precisely because he had smacked lefties in the past, to the tune of 296/388/504 from '02-'04. His numbers this year, then, were about the same: a little more OBP, a little less slugging.

Crosby only had one full year of at-bats prior to '05 and his performance against lefties was awful: 194/287/381. That kind of performance could have easily cost him his Rookie of the Year trophy. That said, the A's probably would have been foolish to expect him to hit that terribly against portsiders in '05, though expecting the excellence he came through with might have been seen as overly optimistic. On the other hand, looking forward, I'd be disappointed if Crosby didn't put up the kinds of numbers against lefties over the next 3-5 years as he did this year.

Eric Byrnes had put up good numbers against lefties in the past as well: 298/361/524 from '02-'04. His power spiked (again, as we might expect for a player his age) even beyond an abnormally high batting average, but this production wasn't so out of line with the past that it could be said to be responsible for the team's supposedly fluky performance against lefties.

Kotsay had hit 301/360/420 against lefties from '02-'04, so his average was better this year, but the overall OPS gain was only 37 points, so we're not talking smack-your-forehead crazy here.

For Dan Johnson, we don't really have anything to go on, but it's not like he went nuts or anything. Given that he's with the A's because of his disciplined, professional-hitter approach, I wouldn't be surprised to see him hit something with 20 OPS of this again next year.

Finally, Jason Kendall hit 299/377/360 against lefties in the last three years. His numbers this year, then, were actually worse. It's a small difference, of course, and quite possibly lessened or erased entirely by park effects, but it's clear that he wasn't driving any unreasonable hitting attack against the lefties.

In all, then, it doesn't appear that any A's had anything like a "fluke" year against lefties that could explain the performance. In fact, it sort of appears that this is what we'd expect them to do going forward, if not perhaps better due to things like Jay Payton picking it up a little bit, Bobby Crosby getting his full complement of at-bats (taking away from Marco Scutaro), and Erubiel Durazo getting his at-bats (assuming the A's bring him back).