Runs allowed per nine innings pitched -- the title says it all. It's basically ERA with the "E" removed.
For example, in 1972, Nolan Ryan allowed 80 runs in 284 innings, giving him a 2.54 RA9.
Runs/IP x 9
Watch: Rick Porcello registers a RA9 of 3.86, giving up three runs -- none of them earned -- in seven innings.
Why it's useful
A pitcher's job isn't to prevent earned runs. It's to prevent runs. That's what this stat analyzes. It's not a perfect evaluation tool for how well a pitcher did his job. But it's a perfect evaluation tool for how well his job was ultimately done. (See the slight difference?)
RA9 is simply a cousin of ERA, and the player rankings for each are more or less the same, with a few minor differences.