GB% and G/F Rate

If you remember back to the stone age, before FanGraphs existed and we had a veritable cornucopia of batted ball stats at our disposal, you may recall a time when pitchers were judged by their ratio of ground balls to fly balls. G/F rate was often used as a tool to describe a pitcher’s type of batted balls allowed. In general, it works fairly well. At some extremes, however, it breaks down.

Take Lenny DiNardo, for instance. He returned to the majors last week, being called up by Kansas City to fill a hole in their rotation down the stretch. DiNardo’s primary skill has always been an ability to rack up groundballs, and his first start of 2009 was no exception. In fact, if we looked at his 8.0 G/F rate, we would think that hitters were pounding the ball into the ground all day.

They weren’t, however. There’s an additional batted ball type – line drives, and DiNardo gave up six of them. When you include those in the balls in play denominator, his GB% is 53.3%, which highlights the fact that he got a majority of groundballs but doesn’t suggest the same crazy performance than an 8.0 G/F rate implies.

Now, this is obviously a tiny sample, and the correlation between G/F rate and GB% is very high. 95% of the time, either one will give you the same answer. But it’s that 5% that G/F rate may lead you astray. Let’s use a slightly more realistic scenario, also from this year.

Who has been more of a groundball pitcher this year – Jason Hammel or Brett Anderson? Hammel has the higher G/F rate, 1.46 to 1.36, though the difference is small enough that you might just conclude that they’ve been basically the same. However, when looking at their overall batted ball profile, Anderson has a 48.8% GB% and Hammel has a 45.5% GB%, which is a big enough gap to say that Anderson has clearly been better at getting ground balls.

Their line drive rates (23.4% for Hammel and 15.4% for Anderson), not included in the G/F measurement, hide the fact that Anderson has been more of a groundball guy than Hammel. In fact, by ignoring the line drives that Hammel is giving up, it actually gives an incorrect answer to the question.

In practical terms, this is more of a current issue with minor leaguers, where we don’t have the same quality of batted ball data, and a pitcher’s G/F rate is still quoted with some regularity. Just think of this as a word of caution – while G/F rates work when line drive rates are equal, line drives are not always equal.