I'd begun writing this as a comment in the Geren thread but I thought it might develop into something that can branch off into a closer look at the relievers as a whole. Please feel free to take a look at the splits that are linked to. I'm getting ready to cover the minor league All-Star Game in Lake Elsinore tonight and just wanted to get something quick up here to look at.

I don't have the specifics of how many times, say, people pitched two days in a row effectively then had a day off and struggled a day after that, but here's something to start with:

Michael Wuertz: days of rest - he's pitched best with 1 day of rest but he's looked more effective with 0 days of rest. It happens to be that both of the homers he's allowed were after 1 day of rest, though. He's thrown the most with 1 or 2 days of rest. Overall May looks like it's been his best month, June his worst. He's on pace to pitch a similar amount of innings in each of the first three months right now.

Russ Springer: days of rest - he's pitched almost the same number of innings from 0-3 days of rest and he hasn't been much different in any situation. He's had the most appearances on 0 days rest, though. I believe he's gone through the "used a lot, then used a little" scenario. However, comparing his numbers by month May is the outlier with April and June better (though he's pitched half as many innings in June so far as he did in April).

Brad Ziegler: days of rest - fairly evenly split in usage between 0-2 days of rest with most of his workload coming on 1 or 2 days off. So far, he's been most effective by far with 1 day of rest. It'd be interesting to see how much that happens to coincide with his first time facing a team in a series. May was his really rough month but that was impacted by him coming down with the plague.

Santiago Casilla: days of rest - this is kind of interesting. He's been used most often on 2 days of rest and that's his worst split. He's pitched more innings on 1 day of rest and that's his best day. Best month this year is April and it's no comparison. May and June have been terrible and he's hardly pitched at all this month.

Craig Breslow: days of rest - going by his A's stats only (I can't link directly to them here, so you get the overall totals for 2009), he's been used the most with 0 days of rest (as many times as the others put together) and it looks like that's where he's been his best. There may be a correlation between that and how he does after a day or two off. His June hasn't been as good as May but it's still respectable.

Andrew Bailey: days of rest - may be a little harder to look at because he didn't start out in the role he's got now (like Ziegler) but he's worked the most by far on 0 or 1 days of rest. His best split is 2 days of rest followed by 0, with 1 day off not as good (though it's his heaviest use by innings and appearances). He had some struggles in May but was still all right, and the other two months have been excellent. Will probably end up with about the same workload this month as the first two (16-17 IP).