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Joakim Soria is off to a blazing start to the 2015 season. He has a razor thin 1.06 ERA through a month and a half of the season and has converted all 12 of his save opportunities.

Soria's success has calmed the nerves of the Tigers faithful, as the bullpen had lots of question marks coming into the season. Joe Nathan's injury caused the situation to get murkier. Would Soria or Nathan close games when the latter returned from injury?

Now, Nathan's elbow has failed him again, and he will miss the rest of the season. No one seems to care as Soria has dominated the late innings.

However, Soria will not continue to pitch this well. Let me explain.

Defense independent pitching statistics

In 2015, ERA is a dinosaur statistic for pitchers. It depends too heavily on defense, the official scorer and the randomness of batting average on balls in play (BABIP). While ERA still infiltrates the best baseball writing, I won't use it in evaluating the Tigers' bullpen.

Instead, defense independent pitching statistics has taught us to focus on peripheral statistics like walks (BB9) and strikeouts (K9) per nine innings. These numbers reflect the skill of a pitcher and tend to persist from early to late season.

In addition, ground ball (GB) rate is important for relievers. Double plays come in handy for escaping jams, such as when Seth Maness of the Cardinals threw one pitch for an inning-ending double play against the Tigers Sunday night.

This article uses walks, strikeouts and ground balls to evaluate the Tigers' relievers. With so few innings pitched this season, I gathered statistics since the beginning of the 2013 season for a bigger sample size.

In addition, Fangraphs puts these numbers together into a single statistic called xFIP (fielding independent pitching), which has the same runs per nine innings pitched interpretation as ERA.

The one statistic Soria can't sustain

For comparison, consider the AL average for relievers in 2014.

AL relievers in 2014: 3.3 BB9, 8.5 K9, 45.3% GB, 3.67 xFIP.

Soria has the following statistics since the start of the 2013 season.

*Joakim Soria: 2.4 BB9, 9.6 K9, 45.9% GB, 3.05 xFIP.

These are great numbers. However, Soria has allowed a .171 BABIP this season, far lower than the .294 average for AL relievers in 2014. This low average has fueled his microscopic ERA.

However, research has shown that pitchers can't sustain that low an average on balls in play. Chris Young, who the Tigers faced recently, is an exception to this rule, but even he has allowed a .248 BABIP over his career.

Soria will not continue to suppress hits at his current pace. But, he's a fantastic pitcher and the best option for the Tigers to close games.

Let's look at the rest of the bullpen.

The set-up guys

*Al Alburquerque: 4.8 BB9, 10.6 K9, 42.9 percent GB, 3.66 xFIP.

This season has started slowly for Alburquerque as he has walked nine batters in less than 13 innings. Moreover, he hasn't compensated for this high walk rate with strikeouts. However, since 2013, he has a better strikeout rate than Soria.

*Joba Chamberlain: 4.1 BB9, 8.2 K9, 46.7 percent GB, 3.81 xFIP.

Chamberlain got rocked in a May 6 outing against the White Sox but has otherwise pitched well this season. He will not strike out as many batters as Soria or Alburquerque but has the highest ground ball rate of any pitcher in the bullpen.

*Tom Gorzelanny: 4.0 BB9, 8.8 K9, 45.2 percent GB, 3.87 xFIP.

Gorzelanny is the best left-handed option out of the bullpen. However, his numbers might look better since he had the luxury of facing pitchers the past two seasons with Milwaukee in the NL.

The rest of the relievers

*Alex Wilson: 2.7 BB9, 6.3 K9, 38.9 percent GB, 4.25 xFIP.

Wilson doesn't walk many batters, but he also has the lowest ground ball rate of any reliever on the staff. His tendency to give up fly balls inflates his xFIP, since this statistic considers an expected home run rate.

*Blaine Hardy (L): 4.6 BB9, 6.8 K9, 45.9 percent GB, 4.56 xFIP.

Hardy has a worse walk and strikeout rate than Gorzelanny, which suggest he's more suited for long relief than late inning match up situations.

The unknown

*Angel Nesbitt: 1.2 BB9, 7.4 K9, 46.4 percent GB, 3.49 xFIP.

With only 14.2 major league innings, it's impossible to say anything definitive about Nesbitt. However, he shows promise on the mound, which gets backed up by his stellar walk and strikeout rate.

Outlook

When I first wrote about defense independent pitching statistics two weeks ago, the Tigers had the 12th worst AL bullpen by xFIP, a runs per nine innings metric. However, they have risen to seventh in the last two weeks. The bullpen will excel if it can sustain this progress.

Ed Feng has a Ph.D. in chemical engineering from Stanford and runs the sports analytics site The Power Rank. Have a question about the Tigers you want addressed in this column? Email Ed Feng here.

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