Ryan Dempster: When ERA doesn’t tell the story

There is a misconception about the way Ryan Dempster performed last year when a trade brought him from the Chicago Cubs to the Texas Rangers.  While there is a lot of truth to the idea that it is more difficult to pitch in the AL compared to the NL, there is also a lot of exaggeration in how league can transform a good pitcher into a bad pitcher overnight.  Now that the Red Sox have signed Ryan Dempster, it seems the rhetoric referring to his “inability to pitch in the AL” has gone much too far.

Dempster pitched 104 innings in the weak-hitting NL Central last season and compiled a stellar ERA of 2.25.  After being traded to Texas and joining a hard-hitting AL West, he pitched 69 innings with a putrid 5.09 ERA.  A night and day difference which seems to confirm the thought that he is a pitcher who thrives in the NL, but gets knocked around in the AL.  The statistical analysis of Baseball has made a lot of progress in the past two decades and has all-but made runs and RBIs irrelevant as well as muting the relevance of batting average as well.  ERA is one more archaic measure that has mostly been left behind in the mainstream acceptance of sabermetrics.  ERA is generally regarded as a very accurate rate statistic for pitcher performance, when in fact there are huge amounts of luck in even year to year values as well as the fact it measures fielder performance as well as being a poor way to compare pitchers from ballpark to ballpark.  ERA, much like batting average, tells us something, but it’s not that exact and it’s not really what people think it’s measuring.

FIP and xFIP have proved to be much more exact statistics when looking at pitcher performance and have also provided a much better tool in projecting future performance.  Both statistics calculate performance based on strikeout and walk percentages, with xFIP also normalizing homerun per fly-ball rates (which are prone to wild variance) to the league average.  Here is how Ryan Dempster performed based on these three statistics for the last 3 seasons.

Ryan Dempster ERA, FIP, xFIP

Dempster holds career rates of 4.33/4.22/3.99 for ERA/FIP/xFIP, so we should expect these numbers to be fairly close on average.  The table shows an erratic ERA from year to year, but very consistent FIP and xFIP.  Upon moving to the AL, Dempster actually raised his K% from 19.9% with the Cubs to 23.3% with the Rangers.  His BB% increased as well, from 6.5% to 8.3%, but still well below his career rate of 10.3%.  His poor ERA in his short stint in Texas can be blamed on a BABIP (Batting Average on Balls in Play), Left on Base %, and HR/FB% well above his career rates.  His increased homerun rate can be explained by pitching in Arlington, known for being a launching pad for baseballs.  His BABIP and LOB% however, don’t make much sense and in all likelihood are just random fluctuations that haven’t evened out in such a short sample.  All these numbers should regress toward his career rates and there is no reason Dempster should be totally useless pitching in the American League.

Ryan Dempster is not going to light the league on fire.  He’s not a top of the rotation starter, and he’s not being paid like one.  He is a pretty decent pitcher being paid to be a league average pitcher with a 2 year $26 million deal.  He’s also a 35 year old pitcher, and being a pitcher of any age means injury-prone, so the risk the Red Sox are taking is quite large, but the other options were either lesser quality or much more expensive.  Once again, Ben Cherington made a move that raised eyebrows based on how boring it was.  There is a perception that Dempster is now a bad pitcher because he is in another league.  It’s disingenuous to judge Ryan Dempster’s ability to pitch in the AL on 12 starts where he put up a 5.09 ERA and think that means anything when he has a proven track record spanning a decade.  The Red Sox needed a pitcher and they got one on a short-term deal without giving up draft picks or prospects.    This deal fits perfectly with what the Sox have done this off-season and provides another component to the “Let’s try to win though” bridge years.



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