From BR Bullpen
A clutch hitter is someone who hits well in key situations. For example, fans may say that a player is a clutch hitter because his hits have won a lot of ball games.
The science of sabermetrics has cast considerable doubt on the concept of the clutch hitter. While a player may hit well in the clutch for part of a season, or even sometimes for a whole season, the effect doesn't last in a statistically meaningful fashion for a whole career. Bill James once did an analysis of Joe Carter's clutch hitting, and found that over a period of many seasons, it averaged out to be essentially non-existent. However, James later explained that the inability of researchers so far to isolate a consistent clutch hitting ability may be because they are not looking in the right places, not because it absolutely doesn't exist.
The flipside of the issue is, if there are clutch hitters, whether there are also clutch pitchers (a term which is hardly ever used). In a seminal 1985 study, researcher Pete Palmer demonstrated through a thorough analysis of won-loss records of every pitcher in major league history with a certain number of decisions that there was no indication of clutch ability. The few anomalies were consistant with the expected distribution of results, which would expect a small number of outliers. An update of that study in 2007 confirmed these results.
 Further Reading
- Bill Deane: "Still Searching for Clutch Pitchers", in The Baseball Research Journal, SABR, Volume 35, 2007, pp. 124-125.
- Phil Birnbaum, Bill James, Pete Palmer and Dick Cramer: "Clutch Hitting", in The Baseball Research Journal, SABR, Volume 37, 2008, pp. 71-88. (a cluster of four articles discussing the state of current research on the existence of clutch hitters, and methodological issues with this research).