Tuesday, October 29, 2013

2013 NPB kwERA Leaders

The simple stat I usually use to evaluate pitchers in the NPB is usually kwERA. With the offensive environment shifting a couple times over the last few years thanks to ball changes, the walk and strikeout rates have stayed relatively the same, making it easier and more effective to use statistics that just use those numbers, not relying on the changing homer and batted ball rates. We could just use strikeout and walk percentages, but using kwERA helps visualize success (or failure) in a run based format like ERA.

The kwERA constant doesn't work the same in the NPB because of a different run environment, meaning, in this case, that NPB kwERAs are pretty consistently higher than ERAs. Therefore, it doesn't really work when trying to figure out which pitchers have been "unlucky" (in simple DIPs terminology), and may be slightly misleading without context. However, I didn't come up with a new constant (the base assumption in kwERA is that a pitcher who walks as many batters as he strikes out "earns" a 5.40 ERA) for two reasons. The first and most obvious, since this is a ranking, we are comparing pitchers to each other. It doesn't really matter what the actual numbers are, just the comparisons between pitchers in the list. Secondly, adjusting the constant might confuse a reader not familiar with the NPB run environment. The kwERA "environment" is similar in both leagues, so seeing a pitcher with a 4.00 kwERA would be a starting pitcher that is good not great, similar to what it is in the Majors.
I also included HR % because, as the name says, kwERA only looks at walks and strikeouts. Without actually factoring it into the formula, looking at how pitchers did preventing homers can be helpful contextually.

In the rankings, I separated leagues because one league has a DH and the other doesn't, which is usually going to affect pitching statistics. However, the Central, which doesn't have a DH, had a higher HR % (driven by Wladimir Balentien) and about the same kwERA, so you could combine the two.

I also labelled each pitcher by which role they occupied during the season in hopes that we could separate the relievers from the starters. Role was determined by majority of appearances. For instance Masahiro Tanaka had one relief appearance, but clearly he was a starter. I used 50 innings as the cutoff for the list.

No surprise, but Masahiro Tanaka lead the NPB in starter's kwERA, followed by Kenta Maeda, Tomoyuki Sugano, Toshiya Sugiuchi, and Chihiro Kaneko. Out of starting pitchers, the best at limiting homers was Daniel Cabrera in the Central League and Shota Takeda in the Pacific League (though Tanaka was right behind him and Takeda had the worst kwERA in the PL out of pitchers with at least 50 innings). Shinya Kayama was the only pitcher in the NPB with at least 50 innings that didn't give up a homer.

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