BB/9 is the control number. Lower is better, and the spread between an elite and a poor mark — roughly three walks per nine innings — is close to a full run of expected damage over a season.

Walks hurt twice. They put a free runner on, and they raise pitch counts, which shortens starts and pushes innings onto the bullpen. A pitcher with good stuff and poor control often has a respectable ERA and a much worse FIP.

That gap is where pitching NUT and ERA part company. NUT charges the walk to the pitcher, every time, no matter how the inning ended.

What is a good BB/9?

Walks are one of three outcomes FIP charges to the pitcher, so a high BB/9 pulls pitching NUT down even when the ERA looks fine.

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