Batting average against is batting average from the pitcher’s side: how often the hitters facing him get a hit. Like batting average, it ignores walks entirely.

It also inherits every problem BABIP describes. A pitcher in front of a strong infield defense will post a lower BAA than an identical pitcher in front of a weak one, and the gap can be 20 points or more without either pitcher throwing differently.

Use BAA as a description of what happened, not as an evaluation of the arm. Pitching NUT, built on strikeouts, walks, and home runs, is the version with the fielders taken out.

What is a good BAA?

BAA is not used in NUT. It moves with the defense behind the pitcher, which is exactly what FIP-based pitching NUT is built to remove.

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