Converting GO/AO to GB%

Because pitcher ground-ball percentages (GB%) are available at FanGraphs and because they strip away the influence of the defense behind a pitcher, they are (to the best of this author’s knowledge) the best available means of adjudging a pitcher’s ground-ball “profile.”

That said, ground-out/air-out ratios (GO/AO) are still more widely available than pure ground-ball percentages — and are, for example, the only grounder-related number Major League Baseball publishes at its site. So it’s not entirely out of the realm of possibility that one could find himself in such a situation as he had access to the one (i.e. GO/AOs) and not the other (GB%s)*.

*In press boxes, for example, stat sheets featuring GO/AO — but NOT GB% — are frequently available.

With a view towards learning more about the relationship between the two metrics, I found both the GB% and GO/AO for the 90 or so pitchers from 2010 with at least 162 innings pitched. Plotting the two against each other (and using a logarithmic best-fit) we get the following:

That’s pretty impressive, it seems, so far as correlation goes.

Using the equation you see there, I computed the expected ground-ball percentages (xGB%) for our 90 qualified pitchers using just their GO/AO ratio.

Here are the leaders:

And here are the laggards:

The expected and actual figures are close enough for this author’s liking — and the fact that the equation mostly holds up in the extremes is satisfying.

Finally, for the sake of reference, here’s a table with approximate equivalencies for GO/AO and GB%:

Note that these equivalencies only hold — so far as I know — for major-league pitchers. That Chris Balcom-Miller, for example, posted a 2.13 GO/AO in 108.2 IP at Low-A Asheville last season does not necessarily mean that he induced grounders on 52% of balls in play. (It should be noted, however, that Chris Balcom-Miller is a future star and you would do well to draft him for your fantasy team or whatever this very second.)