Strikeouts per 9 innings pitched
In baseball statistics, strikeouts per 9 innings pitched (K/9, SO/9, or SO/9IP) is the mean of strikeouts (or Ks) by a pitcher per nine innings pitched. It is determined by multiplying the number of strikeouts by nine and dividing by the number of innings pitched. To qualify, a pitcher must have pitched 1,000 innings, which generally limits the list to starters. A separate list is maintained for relievers with 300 innings pitched or 200 appearances.
The all-time leader in this statistic is Chris Sale (10.84). The only other players who have averaged over 10 (as of August 20, 2018) are Randy Johnson (10.61),Stephen Strasburg (10.55), Kerry Wood (10.32), Max Scherzer (10.35) and Pedro Martínez (10.04).
Among qualifying relievers, Rob Dibble (12.17) is the all-time leader in strikeouts per nine innings. Active leader David Robertson (12.03), Craig Kimbrel (14.7)  and Aroldis Chapman (15.3)  are the only other qualifying relievers averaging more than 12.
One effect of K/9 is that it may reward or "inflate" the numbers for pitchers with high batting averages on balls in play (BABIP). Two pitchers may have the same K/9 rates despite striking out a different percentage of batters since one pitcher will pitch to more batters to obtain the same cumulative number of strikeouts. For example, a pitcher who strikes out one batter in an inning, but also gives up a walk or a hit, strikes out a lower percentage of batters than a pitcher who strikes out one batter in an inning without allowing a baserunner, but both have the same K/9.
- "K/9IP All Time Leaders". Baseball-Reference.com.
- "K/9IP leaders, minimum 200 appearances". Baseball-Reference.com.
- "K/9IP leaders, minimum 300 innings pitched". Baseball-Reference.com.
- "Rob Dibble Statistics". Baseball-Reference.com.
- "David Robertson Statistics". Baseball-Reference.com.
- Craig Kimbrel Stats | Baseball-Reference.com
- Aroldis Chapman Stats | Baseball-Reference.com
- James Gentile (October 8, 2012). "Stop using K/9 and BB/9!". Beyond the Box Score. SBNation.com.
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