Tuesday, March 27, 2012

What Is a Ground-Ball Pitcher?

Earlier today, Rob Neyer wrote a post about how to define a ground-ball pitcher. His conclusion:
More than 50 percent, and you're a ground-ball pitcher. More than 55 percent and you're an extreme ground-ball pitcher (if not quite Brandon Webb).
Well I wasn't completely convinced, so I decided to do this for myself. I used Baseball Prospectus's ground ball statistics and defined GB% as GB/Batted Ball (as they do). Here is a graph of GB% since 1950:

Click to enlarge

The data from 2000-2002 only show outs, so those years are removed from this analysis. Throughout the 50s and 60s, GB% rose steadily. It held fairly constant until the early 80s, declined through the 90s and has been holding steady again throughout the 2000s. So the definition of a high ground-ball pitcher has changed throughout the years.

And how did I decide to define a high ground-ball pitcher? With z-scores, of course. For the more inquiring minds out there, here is a Q-Q plot for the 2011 data, including pitchers with more than 100 batted balls:

Click to enlarge

Those three outliers are Jonny Venters, Brad Ziegler and Bobby Cassevah. The data do appear to have a normal distribution, so I'll go ahead with the z-score analysis.

Very low to very high ground-ball pitchers are split up by standard deviation. Very low is 2 standard deviations below the mean and Very high is 2 SDs above the mean. Low and high are 1 SD above or below the mean. This table shows these percentages for pitchers for every year since 1950, excluding 2000-2002:

Click to enlarge

And here are the most extreme ground-ball pitcher years. I filtered the results to show only pitchers with more than 300 batted balls, just to make it more interesting:

My conclusion? Over the past 25 years, a high ground-ball pitcher has a GB% over 54. An extreme ground-ball pitcher's GB% is over 61.

If you have any questions, you can contact me in the comments below or on Twitter @stealofhome.

1 comment:

William Tasker - Caribou, ME said...

Cool stuff. Thanks for adding to Neyer's narrative.