Clutch measures how well a player performed in high leverage situations. It’s calculated as such:
In the words of David Appelman, this calculation measures, “…how much better or worse a player does in high leverage situations than he would have done in a context neutral environment.” It also compares a player against himself, so a player who hits .300 in high leverage situations when he’s an overall .300 hitter is not considered clutch.
Clutch does a good job of describing the past, but it does very little towards predicting the future. Simply because one player was clutch at one point does not mean they will continue to perform well in high-leverage situations (and vice versa). Very few players have the ability to be consistently clutch over the course of their careers, and choking in one season does not beget the same in the future.
The majority of players in the league end up with Clutch scores between 1 and -1, with zero being neutral, positive scores being “clutch”, and negative scores being “choke”. Only a few players each year are lucky enough (or unlucky enough) to have extreme Clutch scores.
Links for Further Reading: