from Amazon
Wednesday, October 21, 2009
Starter v Relief, 1953-2008
Rally compiled this great file. It has each pitcher’s starter and relief numbers lined up by pitcher/year. He also shows you extra columns where all the data is equally weighted for each pitcher. Fantastic stuff, and EXACTLY the way I would have done it. This explains why I think it’s great anyway! It’s one of those things I’ve been meaning to do since forever. Rally compiled the data for all of us to use.
Here’s what I find. I went ahead and calculated the wOBA for all the starters/relief, and for pitchers since 1993, it was .352 for starters and .323 for relievers, a gap of 29 wOBA points, almost identical to what I found in The Book. The relievers had a wOBA that was 8% less than starters. When I looked at the whole dataset (1953-2008), it was also the same 8%. Basically, this reafffirms the 1 run per 9IP gap I’ve been using.
You can try slicing and dicing Rally’s data any way you like, but you will see little bias in eras. Basically, use the “rule of 17”: difference in BABIP is 17 points higher as starter. K/PA is 17% higher as reliever. And HR per contacted PA is 17% higher as starter. Walk rate is FLAT.
I know about the selection bias. It was discussed at length in The Book. (For those who don’t have it, read it for free on Amazon.com.) The next step is to do what I did in The Book, for all these pitchers. I’d bet you’ll find results that match those.
The golden rule basically comes down to this: when you compare a reliever’s stats, you MUST MUST MUST use a different baseline than a starter. And that baseline is roughly a 1 run difference per 9IP. And it has nothing at all to do with leverage. The basic way to think about it is that a starter is pitching with one hand tied behind his back. So, you can’t compare him straight up to a reliever.