Tuesday, March 29, 2011

Jeremy Affeldt's Outlier

Jeremy Affeldt is a good relief pitcher, but many people are distracted by one thing; 1.73.

That was his ERA in 2009.

His next best season was a 3.33 ERA season. This is not to say that pitchers can't put it together all of a sudden and become dominate forces. I'm saying that this was an outlier and a deviation from his true talent level.

If you look at his last three seasons and look at the pitching stats that try to strip out things beyond a pitcher's control, it becomes fairly obvious that one season does not look like the rest.

Year BB/9 SO/9 SO/BB
2008 2.9 9.2 3.20
2009 4.5 7.9 1.77
2010 4.3 7.9 1.83
Provided by Baseball-Reference.com: View Original Table
Generated 3/29/2011.

If you take a glance at his peripheral stats, you would expect that his 2008 should have been the best season, which it was according to the advanced pitching statistics. However, 2009 is looked at as his superior season because of the sparkling ERA. What isn't seen is that he had career years in BABIP, strand rate, ground balls, and home run rate that were all significantly below his career norms.

What this means is the Affeldt of 2009 is most likely a mirage. It was amazing while it was happening, but it wasn't his true talent level.

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