from CD-ROM game help section:

Clutch hitting measures each player's ability to deliver the clutch hit. Strat-O-Matic's exclusive clutch hitting formula accurately measures every player's ability in these key situations. Traditional clutch hitting stats do not comprise enough situations to make them statistically significant. For instance a batter with 500 at-bats might only hit 110 times with runners in scoring position and perhaps 80 times in "close and late" situations. This total number of at-bats is not enough to truly gauge his clutch hitting ability, because in 190 at-bats a .250 hitter can be expected to swing anywhere between .187 and .313 just as a result of random statistical distribution. Strat-O-Matic's exclusive clutch hitting formula measures the outcome of every baserunner that the batter affected during the year. This detailed approach provides a true measurement of the batter's clutch hitting ability. Our system is implemented with 2 outs and runners in scoring position because in real-life that is traditionally the most difficult time to "come through" with a base hit. If the batter fails, there are no more chances to drive in those runners. Most batters will see their average go down in the clutch -- both in real-life and with our system.