## K% and GB% standard deviations and their effects on performance

A guy with strikeout and ground ball skills. He's made his dent in history. - USA TODAY Sports

If we can control for one variable, can we discover which affects performance more?

In this installment of my standard deviation starting pitcher series, I will be examining the relationship between K% and GB% and how those two variables affect performance. The methodology is basically the same as before; since batted balls are included, the years included in this analysis are 2002-2013. In addition, only starting pitchers who have at least 70 batters faced for a stable K% and 70 balls in play (GB+FB+LD) for a stable GB% are included. I have again chosen RA9-WAR as the performance metric for each group.

 GB% -2 -1 0-1 0+1 1 2 RA9-WAR K% -2 N/A N/A -1.1 N/A -0.6 -0.7 -0.7 -1 N/A -0.5 -0.1 -0.1 0.2 1 -0.1 0-1 0 0.3 0.4 0.7 1.1 1.9 0.7 0+1 0.6 1.2 1.4 2 2.7 2.8 1.7 1 1.1 1.9 2.7 4.1 3.3 5.8 3 2 N/A 2.4 5.5 4.5 3.7 N/A 4.8 RA9-WAR 0.5 0.9 1.1 1.4 1.6 2.3

Across the top of the table are the GB% standard deviation groups, and going down the table are the K% standard deviation groups. -2 means at least two standard deviations below the middle, -1 means at least one standard deviation below the middle, 0-1 means between 0 and 1 standard deviations below the middle, etc. The RA9-WAR values shown in the far right column are the median RA9-WAR values for each K% group regardless of GB%, and the RA9-WAR values shown in the bottom row are the median RA9-WAR values for each GB% group regardless of K%. For example, the median RA9-WAR for all pitchers who were at least 2 standard deviations above the middle K% is 4.8. N/A means there were no pitchers in that group.

Compared to the K% and BB% relationship, the relationship between K% and GB% is not quite as clear. It seems that at the low end of the K% spectrum, having a higher GB% can help overcome a low K%. For example, the group whose K% is between 0 and 1 standard deviations below the middle and GB% is 2 standard deviations above the middle, abbreviated (0-1,2), has an RA9-WAR value of 1.9. The RA9-WAR value for the K% of this group is 0.7, so having a higher GB% aided that group's performance. The lower K% does bring down that group's performance compared to all pitchers who were at least 2 standard deviations above the middle in GB%, though, as that group's median RA9-WAR was 2.3.

On the higher K% side of things, there were mixed results. Those pitchers in group (1,2) had a median of 5.8 RA9-WAR, though only six pitchers are in that group. The combination of high K% and high GB% elevated the performance of that group above the median RA9-WAR value for either K% alone or GB% alone. However, group (1,1) had a median RA9-WAR value of 3.3, which does not follow the general trend of increasing RA9-WAR with increasing GB% and wasn't far from the overall K% +1 group median RA9-WAR value of 3. There were 43 pitchers in that group, so sample size doesn't appear to be the cause.

In general, having a higher GB% can help elevate the performance of a pitcher regardless of K%, but this relationship is not as clear as K%. Having a higher K% can overcome a low GB%, but there are situations in which a lower GB% can be a good thing (see the Oakland Athletics). It appears that the effect of K% is stronger than the effect of GB%, but having a higher GB% is a good way to extract value out of cheaper pitchers whose K% is low.

. . .

All statistics courtesy of FanGraphs.

Kevin Ruprecht is a contributor to Beyond the Box Score. He also writes at Royal Stats for Everyone. You can follow him on Twitter at @KevinRuprecht.

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