The Truth About the Grounderby David Gassko
May 12, 2006
Ground ball pitchers have suddenly become the biggest commodity in baseball, it seems. With baseball statisticians discovering the importance of keeping the ball out of the park (not that this wasn't an obvious fact before—just perhaps not thought to be as important as we now know it is)—and the great correlation between that and keeping the ball on the ground—ground ball pitches have become in-demand, and highly hyped. With many sites now tracking ground ball and fly ball information for the minor leagues, pitchers that induce grounders at a high rate have started to receive praise that is often not in-line with their numbers, or abilities.
Partly, this makes sense. Here at THT, we purchase batted ball data from Baseball Info Solutions because we believe that this data will play a large role in the future of baseball analysis. Batted ball data strips away a lot of luck from player performance, and is thus more reliable, especially over small samples. Minor leaguers generally do not have a large performance history, given their limited pro experience, and the shortened minor league season, so batted ball data can be especially helpful when evaluating players from the minor leagues. And yet, it's being taken to overly great extremes too often lately.
Take, for example, Jered Weaver. Through Sunday, only 25% of the balls hit off him had been hit on the ground. Then again, he was pretty much striking everyone and walking no one with a pretty average home run rate. He's been that way his whole career. Yet, "serious" baseball analysts everywhere dismiss him, simply because of the ground ball rate, a sure sign that Weaver will not succeed in the major leagues. Now I don't whether or not he’ll pitch well in the big leagues, and I don’t know how worried we should be about the ground balls.
But what I do know is that there's a lot of talk in the baseball community about ground balls without much actual knowledge. That's what interests me: The truth about ground balls.
Ground Balls and Home Runs
The main advantage of a ground ball pitcher is supposed to be his ability to prevent balls from leaving the park. The basic idea is simple: A pitcher has little to no control over the rate at which his fly balls leave the park, as was shown in my study with JC Bradbury in The Hardball Times Annual 2006, however, as we also showed, pitchers exhibit very stable ground ball rates from year-to-year. So if the only thing a pitcher can do to prevent home runs is to prevent fly balls, well then, it stands to reason that ground ball rate is a key factor in preventing home runs.
However, many have argued that this is not quite correct, because pitchers with high ground ball rates generally only allow fly balls when they make a mistake, and since mistake pitches will be hit farther than your average fly, ground ball pitchers should allow more home runs per fly ball than your average pitcher. Is it true? The numbers say no. The correlation between a pitcher's ground ball rate, and the percentage of the balls hit in the air against him (measured as outfield fly balls plus line drives) that land in the seats is -.05, otherwise known as non-existent.
The closer a correlation is to 0, the less of a relationship two variables have. In this case, the percentage of balls hit off a pitcher that go on the ground and the percentage of balls hit in the air against that turn out to be home runs have absolutely no relationship.
Ground Balls and Line Drives
While the theory that ground ball pitchers will allow more home runs per ball in the air because those balls are mistakes is incorrect, that does not mean that the theory is unreasonable. In fact, it sounds downright logical, and while logic and baseball often don't mix, in this case, we may be able to find a happy marriage. Home runs are just one type of mistake, the other is line drives.
Line drives are probably the most interesting type of batted ball stat to study, because on one hand, they have an astronomical run value, and on the other, skill in preventing line drives is very hard to detect. In fact, JC and I found no year-to-year correlation in terms of the percentage of batted balls that are line drives for pitchers, meaning that it's almost impossible to predict how good or bad a pitcher will be at preventing line drives.
Well, it may be more possible than we think. There is, it seems, a very strong correlation between the ground ball rate and the percentage of balls in the air (outfield flies plus line drives) that are line drives—.67 to be exact. What does that mean? Simply put, ground ball pitchers are much more likely to allow a line drive on a non-ground ball, and vice-versa. Logically, that makes sense: For a fly ball pitcher, a fly ball may well be a success, for a ground baller, it's almost surely the result of a mistake.
This tells us a lot of about line drives. While overall, they may be unpredictable, by using this type of analysis, we can find ways to indeed get a better feel for which pitchers are expected to allow more or less line drives. Of course, the whole issue is muddied up by the fact that ground ball pitchers allow less balls in the air overall, so their line drive rates will be closer to (though still higher than) average, even if they're more likely to allow a liner when the ball is hit in the air.
What this finding also means is that ground ball pitchers do not have as much of an advantage over fly ball pitchers as some might think. Being a ground ball pitcher may help suppress home run rates; it does nothing for line drive rates.
Ground Balls and Unearned Runs
I've written about this a little before, but since some apparently took me seriously, I felt the need to examine this relationship a little more. The fact is, the idea of earned runs is arcane given all we know about defense, and the plethora of data we have for modern years. Earned runs are supposed to split runs between the pitcher and his defense, but the fact is, there are better ways to do this. One way to prove this is to look at the relationship between unearned runs and ground balls.
And the relationship is a strong one. The correlation between the percentage of a pitchers runs that were unearned and his ground ball rate is .33, which means that ground ball pitchers are expected to allow more unearned runs than fly ball pitchers. Of all ground balls, 2.23% end up as an error, which accounts for 85% of all errors overall. Since ground ball pitchers are so susceptible to errors, they're bound to see a lot more unearned runs. It's not bad defense that's resulting in those runs being unearned, but rather the pitcher's ground ball tendencies.
What this means is that we might actually overrate some ground ball pitchers on the basis of their ERAs. Take Derek Lowe, for example. His ERA last year was 3.61, a respectable 13% better than league average. It's easy to be deceived into thinking he had a good year last season, if you don't take the unearned runs into consideration as well. However, if you do, Low was actually 7% worse than league average. When it comes to ground ball pitchers, ERAs can be deceiving.
What I don't want people to think after reading this article is, "ground ball pitchers good" or "ground ball pitchers bad" or anything like that. Because neither is true. There are certain advantages to having a high ground ball rate—like a low home run rate—and certain disadvantages, like a high line drive rate and more unearned runs. The fact is, just like any other pitcher, ground ball pitchers can be successful and unsuccessful, based on how well they do other things, like controlling the strike zone. But the ability to prevent balls from being hit in the air itself is not all that important; certainly not as much as some would like you to believe.
David Gassko is a former consultant to a major league team. He welcomes comments via e-mail.