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What is tRA?

Posted By Graham MacAree On August 14, 2009 @ 3:30 pm In Glossary | 28 Comments

As I’m sure many of you have noticed, tRA has come to FanGraphs. I’m going to try to describe it as concisely as possible, so here goes.

tRA can be seen as an extension of FIP including batted ball types, namely line drives, ground balls, and the different types of flyballs. The idea of using these is to separate defense from pitching while still incorporating some measurement of how ‘fieldable’ the contact a pitcher induces is. Line drives are a little trickier for defenders to handle than ground balls, and tend to lead to more runs scoring. This is reflected in the runs/outs data we have for batted ball types, which leads the way quite neatly to tRA.

If you aggregate the tRA outcomes (K, BB, HBP, HR, + batted balls), and apply run/out values, you end up with expected runs (xR) and expected outs (xO). We can easily convert this to runs per nine innings by taking xR/xO*27. That’s tRA. Note that it is not on the familiar ERA scale, as I believe a defensive neutral statistic should expect defenders to have a league average error rate. League average tRA is typically in the high 4s.

Why use tRA? Well, it’s an interesting tool to supplement FIP with if you want to look at how hard a pitcher is being hit. It’s not a FIP killer by any means, and the difference between StatCorner tRA (using MLB AM classifications) and FanGraphs tRA (using BIS classifications) should tell you why: batted ball types are pretty subjective. However, they’re not wildly inaccurate, and using tRA, especially alongside pitcher batted ball information will give a better understanding of what exactly a pitcher is doing.

I’m very pleased tRA has made its way to FanGraphs, and I can’t say enough thanks to David Appelman for making it possible. I hope everyone finds it useful.


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