Analyzing baseball yields many numbers of interest and value. Yet far and away -- far, far and away -- the most critical number in all of baseball is 3: the three outs that define an inning. Until the third out, anything is possible; after it, nothing is. Anything that increases the offense's chances of making an out is bad; anything that decreases it is good. And what is on-base percentage? Simply yet exactly put, it is the probability that the batter will not make an out. When we state it that way, it becomes, or should become, crystal clear that the most important isolated (one-dimensional) offensive statistic is the on-base percentage. It measures the probability that the batter will not be another step toward the end of the inning.