Entering Wednesday's game, Mark Buehrle had held opposing batters to a .268 batting average throughout his career. Assuming he performs at his career average level, how often will he pitch a no-hitter?
Ignoring for now the effects of fatigue, walks, hit by pitch, etc., Buehrle will retire 27 consecutive batters without a hit with probability (1-.268)^27. That works out to a no-hitter once in every 4552 starts, or about once every 140 seasons. Rarified air indeed.
By the way, a similar analysis leads to one no-hitter per 2198 starts for Greg Maddux, 1082 for Roger Clemens, and 561 for Pedro Martinez. These three are often cited as proof of how hard it is to record a no-hitter, although this is a crock for Pedro, who was screwed out of a perfect game when his team failed to push a run across in 9 innings of regulation.
Not to take anything away from Buehrle's achievement, but he is no Clemens, Maddux, or Pedro, even if he outshined them for one night.
What's really scary is that Nolan Ryan averaged nearly one no-hitter per 100 career starts. This is one of baseball's few records that is in no real jeopardy of being broken.