Converting GO/AO to GB% (Retrosheet Remix)

Last Friday, I submitted for the readership’s consideration a brief post on how one might convert the ground-out/air-our ratios (GO/AO) found at MLB.com to the ground-ball rates (GB%) found here at FanGraphs. Though, as the much esteemed Tangotiger noted, the work wasn’t entirely grounded (get it?) in logic, the effort satisfied my immediate concern — namely, to create a quick-reference table for translating MLB’s GO/AOs (which are, for example, sometimes included with press-box stat sheets) into the GB%s with which most saber-oriented readers will be more familiar.

Of course, MLB.com is not the only site that publishes GO/AO data. Retrosheet (via Baseball-Reference) has GO/AO ratios going back to 1950. If it so happened that Retrosheet’s GO/AO numbers correlated strongly with our GB%s here, then we might — and I’ll stress might — have a tool with which to look back at some 60 years’ worth of ground-balling data.

To test the correlation between Retrosheet’s GO/AO and GB%, I took every qualified pitcher season from 2002 to 2010 (i.e. the years for which we have GB% data).

Here are the results:

The reader will note that the correlation coefficient here is actually higher than from the single year of MLB data we looked at on Friday.

The reader will also note that Retrosheet’s GO/AOs are generally lower than MLB’s — averaging 1.18 over 2002-10, where the average GO/AO for qualified pitchers in 2010 per MLB was 1.45. This appears to be consistent with the fact that — among other factors — MLB omits line-outs from its data, thus decreasing the denominator.

Calculating the expected ground-ball percentages (xGB%) for all the pitcher seasons in the data set using the equation from the above graph, we find that the root mean square error (RMSE) for said xGB%s comes out to a mere 1.4%. That seems pretty good.

As with last time, here the leaders by GB%:

And the laggards:

Finally, here’s the quick-reference table for the approximate equivalencies between Retrosheet’s GO/AO data and our GB%s:

As I note above, it’s possible that — owing to the strength of the correlation between the GO/AO and GB% data — that we might be able to make some reasonably confident statements about ground-ballers prior to 2002. That will be an area of focus in my next post.