There were a couple of great studies updated on the Retrosheet web site by Tom Ruane recently. The first deals with clutch hitting since that's been a topic of some interest recently. I like this study because it uses much larger sample sizes than the original clutch hitting studies that simply look at year to year differentials.
Tom presents a mountain of data from 1960-2004 in a very understandable format. I was particularly interested in his data that shows that hitters do indeed perform more pooly with runners in scoring position than they do in other situations once sacrifice flies, hit batsmen, and walks are taken into account. He also breaks the data down into hitters who had above and below 3000 clutch at bats.
1-2999 AB H 2B 3B HR BAVG SLG BPS
Not RISP 1748765 414965 70688 9459 35513 .237 .349 .587
RISP 588724 135614 23479 3614 10466 .230 .336 .566
>=3000 AB H 2B 3B HR BAVG SLG BPS
Not RISP 2848124 773189 135086 19045 84334 .271 .421 .693
RISP 974948 257380 45517 7173 26300 .264 .406 .670
There are two impliciations here. First, you would expect that hitters with more clutch at bats would have a smaller differential between non-clutch and clutch performance on the theory that these hitters were "selected" for more at bats in part because of their ability to perform when needed, but this was not the case. They were better hitters of course but saw the same dropoff in clutch situations. Seconodly, this does provide support for the notion that if clutch hitting exists the defintion of a clutch hitter is more a hitter whose performance does not decline in key situations but rather remains the same or slightly improves.
Digging into this second point Tom also explores some plausible theories as to why performance might fall off in these situations including the ability for the defense to record force outs, the spike that occurs when a runners is on first due to the first baseman holding the runner, the fact that better hitters are walked more often with a runner on second or second and third, platoon advantages, quality of pitching, and even park factors. He found that platoon advantage for the hitter and higher quality of pitching tend to balance each other out in the long haul. In looking at his data I found that when excluding the at bats with a runner on first only the difference shrinks to around 11 points of BPS (batting average + slugging percentage), roughly half of the raw difference. As a result it would appear that hitters do tend to hit more poorly in clutch situations overall.
In conclusion Tom reports:
"So did I find evidence of clutch hitting? Not really. I did come up with lists of players who performed well and poorly in this area. Along the way, I presented quite a bit of data on situational hitting, platoon advantages, opposition pitching strength and park effects, and I attempted to both understand and explain what I found. At the end of all this, however, I guess I'm still not convinced that the players owe their inclusion on these lists of mine to talent rather than luck. Even when dealing with sample sizes of several thousand at-bats, the amount of random variation that I found in my simulations was very close to what I found in the real data. As I mentioned before, this doesn't necessarily mean that there isn't some real differences buried in all that noise, only that I'm not sure I found them. One could argue that the forces at work here, if they exist, must be awfully weak to so closely mimic random noise, and if they are really that inconsequential perhaps we could assume they don't exist without much loss of accuracy."
To me this tends to confirm what most sabermetricians have been saying since 1978 - clutch hitting may be a real skill but if it is it makes very little difference in the long run.
The second study is an update to an approach discussed in the 1987 Baseball Abstract. Tom applies a slightly different methodology and also includes positional adjustments and stolen base runs. He compares his method with a more straightforward linear weights method and even applies the method to pitchers. Very interesting reading.