This year, we’ve seen the Rays bullpen struggle intermittently, driving us all insane because sometimes they’re lights out and other times they give up 8 runs. But something interesting might actually be happening for relievers across baseball this season. Let’s take a look at that.
This season (entering Thursday), relievers across Major League Baseball have accumulated a 171-173 record, 2 games under .500. In 2011, they were 713-702, 11 games over .500, and in 2010 they were 694-677, 17 games over. Major league relievers haven’t been under .500 since 2008. But then again, it’s early in the season and a lot could and will change. Relievers are also becoming pitchers of records in more games, with 10.8% of their appearances ending with a decision up from 10.2% in 2011 and 9.8% in 2010. But what’s so strange is their sub-.500 record despite the way they performed. MLB relievers have posted a 3.66 ERA in 2012, right around the 3.69 mark from 2011 and much better than the 3.93 MLB mark from 2010. What is going on here?
The knee-jerk reaction is to check RA, run average, which factors in unearned runs. 2012 MLB relievers have a 4.01 RA, quite a bit higher than their ERA, but the relievers’ RA in 2011 was 4.02 and in 2010 was 4.26, so that’s a dead end. Even using FIP, which has no direct bearing on past performance but is useful in predicting future performance, the 2012 reliever mark is 3.85, the same (to two decimal points) as 2011 and a bit better than the 3.96 mark relievers posted in 2010. Since the 2012 relievers are clearly just about as good as 2011 and a bit better than 2010 overall, there’s clearly some variation going on here. A losing record might mean that a few teams have had terrible bullpens this year, and maybe some of the better bullpens play on teams with great starting pitching that limits the opportunities for their relievers to earn wins. Let’s see if that’s the case.
The average ERA for MLB bullpens in 2012 was little bit higher than the total ERA, coming in a 3.68 with a standard deviation of .878 runs per 9 innings. Let’s compare this with 2011 and 2010. Before we even get to that though, we have to acknowledge that there’s a lot of regression to the mean that still has to occur in 2012. Thus far in 2012, there are 8 teams with bullpen ERA’s under 3.00 and 3 with ERA’s over 5.00. In 2011, there was not a single team who fit either of those categories while in 2010 there were two teams with ERA’s under 3.00 and one with an ERA over 5.00. Here’s a comparison of the five number summaries for all three years, which gives us a better idea in terms of how to compare them.
For those of you who are unfamiliar, unlike the mean or average that is affected by outliers, the median is just the middle number or in this case, the average of the 15th and 16th place teams in baseball in terms of ERA. The other numbers give us an idea of the spread of the data- the first quartile is the end of the first quarter of the numbers or the middle number of all the values less than the median while the third quartile is the end of the third quarter of numbers or the middle number of the values greater than the median. An outlier is defined as greater than 1.5 times the interquartile range, which is the third quartile minus the first quartile, greater than the third quartile or 1.5 times the IQR less than the first quartile. The only outlier among the three years is the Arizona Diamondbacks’ horrific 2010 bullpen, which posted a 5.74 ERA. (Incredible that they made the playoffs last year.)
We see that the 2012 data has a much higher range of values than 2011 and even 2010. Its minimum value and first quartile are lower than the other two years, but its median is actually slightly higher than 2011 and its third quartile and maximum value are much higher than 2011 and 2010 when we take out the outlier in the 2010 data. The reason that the 2012 bullpens have a losing record is that even though there are whole bunch of teams with excellent bullpens, the rest of the bullpens have clearly slipped. There are no teams with a bullpen ERA between 2.93 and 3.30.
Keep in mind that the sample size in 2012 is just 3327 innings compared to 14228 in 2011 and 14238.1 in 2010. It makes sense that there’s a lot more variability. In 2011, there was a mean ERA of 3.69 and a standard deviation of just .397 runs per 9 innings. In 2010, the mean ERA was 3.94 with a standard deviation of .591 runs per 9 innings, and that’s even with the outlier included. Taking the outlier out, that slips to .492. A lot of evening out is going to happen through the rest of 2012. Most bullpens, even the best ones and some of the worst ones, are going to end up in the 3.00′s in ERA. There is currently a .417 correlation between ERA and FIP, meaning that as ERA went up, FIP tended to go up but with plenty of exceptions. A more staggering correlation is the .855 correlation between ERA and ERA minus FIP, essentially telling us that the teams with the lower ERA’s are going to be in for a rude awakening as their ERA’s regress towards the FIP’s while the teams with higher ERA’s aren’t quite as bad as they seem. The teams in big trouble: the Orioles (2.18 ERA, 3.74 FIP), the Pirates (2.63 ERA, 4.07 FIP), and the A’s (2.74 ERA, 4.03 FIP). The Brewers (4.88 ERA, 3.45 FIP) the Mets (5.13 ERA, 3.99 FIP), and the Tigers (5.17 ERA, 4.13 FIP) have reason for optimism. The Rays have a 3.89 ERA compared to their 3.78 FIP, meaning they’re due for some improvement- and considering their defense hasn’t played great yet this year, once their defense gets on track their bullpen ERA could go down considerably.
Is 2012 a particularly notable year for MLB bullpens? Probably not. There are a bunch of teams with bullpens pitching out of their minds while several teams’ bullpens are struggling, but by the end of the year it should all even out. For the Rays and most of baseball, that’s a good thing.
Topics: Statistical Analysis