H/9 represents the average number of hits a pitcher allows per nine innings pitched. It is determined by dividing a pitcher's hits allowed by his innings pitched and multiplying that by nine. It's a very useful tool for evaluating pitchers, whose goal is to prevent runs, which are usually scored by hits.
Though it's closely correlated with opponents' batting average, hits per nine is based solely on the number of outs a pitcher records, rather than the number of at-bats by his opponents. This is important because H/9 takes into account the second out on double plays, sacrifices and other outs that occur without an official at-bat being recorded -- such as outfield assists. Walks, hit by pitches and other means of reaching base do not play a factor in H/9.
The best pitchers in baseball typically feature the lowest H/9 marks, however there are a few flaws in using the metric out of context. A home run allowed is far more harmful to a pitcher than a single, yet they all count the same in H/9. Also, the sequencing of hits does not play a part. If a pitcher throws three flawless innings, then allows four straight singles in the fourth inning, he'll generally allow at least one run. But if he allows a single per inning over four innings, chances are he won't allow a run. Yet, in both cases, his H/9 is the same.
Watch: Tyson Ross allows a hit, raising his hits per nine innings.
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