How imperfect observation ruins a potentially perfect game

On Wednesday, Detroit Tigers pitcher Armando Galarraga was moments away from a near perfect game, only to be foiled by a questionable call. Without that call, Galarraga would have been the third pitcher to throw a perfect game in as many weeks. What gives?

Believe it or not, perfect games happen more often than you think. Marc Normandin, writer for Baseball Prospectus, has a few ideas about the factors that go into pitching a perfect game.