Get to Know: K/9
K/9 (strikeouts per 9 innings): The average of how many batters a pitcher strikes out per 9 innings pitched.
Calculated as: (SO * 9) / IP
Why you should care: K/9 is a perfectly suitable way to evaluate a player’s ability to strike batters out.
Current Baselines (2002-2007): The average K/9 for starting pitchers is 6.17 and 7.21 for relievers. For starting pitchers the top and bottom 20th percentile are a K/9 above 7.56 and below 4.89. Relievers top and bottom 20th percentiles are a K/9 above 8.94 and below 5.54.
Variations: Some people prefer to use strikeouts per batter faced (K% or K/G) to express a player’s ability to strike batters out. The difference is minimal and the argument for using K% is that K/9 excludes walked batters and K% does not, suggesting that K/9 may either overstate or understate a pitcher’s overall effectiveness (not pure strikeout ability).
Links and Resources:
Wikipedia: Strikeouts per 9 innings pitched
U.S.S. Mariner: Evaluating Pitcher Talent
studes said,
March 13, 2008 @ 8:49 am
I think the rationale for K/BFP (at THT, we multiply BFP by 38, or the average number of batters per game in a given league in a given year) is stronger than that. Good pitchers are definitely underrated by K/9, because they face less batters per nine innings (that’s not just walks, but hits allowed, too). I guess the difference is “minimal,” but that depends on your expectations, right? Also, the differences are biased — good pitchers are underrated and bad pitchers are overrated — which makes K/9 less insightful and more misleading.
David Appelman said,
March 13, 2008 @ 10:54 am
I don’t think that makes K/9 is any less insightful as long as you know what you’re using. In all honesty they are really insanely similar with a r-squared of .96. The biggest difference you’re ever really looking at is not much more than 1 K per 9/G.
But, with that said, the players who have their K/9 most “undervalued” compared to K/G were typically those who had pretty good or great seasons. And likewise players who were most “overvalued” were players with poor seasons.
With K/G there’s a slight possibility (and I do mean ever so slight) you may be missing out on a potential breakout pitcher since it undervalues pitchers with high K/9 that walked a lot of batters and gave up a lot of hits. It’s tough to say K/9 undervalues players that already have K/G up in the 9+ range which is already quite high.
I don’t think either of these stats should be used in isolation, but if you had to use one in isolation I probably would use K/G. Assuming I had the rest of the stats on hand like BB/9 (or whatever flavor of that you want) along with HR/FB etc… I’d probably stick with K/9.
studes said,
March 15, 2008 @ 7:47 am
Not to drag it out, but why exactly do you prefer K/IP over K/BFP?
David Appelman said,
March 15, 2008 @ 9:08 am
I think it isolates strikeout potential better than K/BF does.