John from the inimitable Twins Geek blog introduced me today to a stat called ARP, which seems pretty obviously to be far and away the best measurement of reliever effectiveness. I’m going to post an excerpt here, but I encourage you to check out his entire explanation. (And while you’re there, add him to your RSS.) Very, very enlightening stuff:

So say a reliever comes in with runners on the corners and no outs. From studying tens of thousands of previous MLB innings, we know, on average, how many runs will score that inning. It’s about 1.9. So if the reliever strands those runners, he gets +1.9 ARP. If he gets out of the inning with just one run scoring, he gets +.9. If two runs score, he gave up more runs then the average, so his ARP is -.1.

Or say a reliever comes in to pitch the ninth inning. The average runs scored in an inning are .56. So if he pitches a perfect ninth, he gets +.56 ARP. And if he gives up a 2-run home run to Alex Rodriguez (for example) his ARP would be -1.44.

Sold. I am completely and utterly sold. I hope you are too, because I’m not looking at or referencing reliever ERA anymore. (OK, that was just said for effect. I never looked at reliever ERA. Just trying to make a point here.) Come on, baseball fans, join me aboard the revolution.

(Here is the full ARP list from 2009.)

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