One of the problems with statistics such as ERA and FIP is that they do not consider the impact of situations in which a pitchers pitched. This is a problem for evaluation of relief pitchers because the influence of their innings on the outcomes of games varies so much by situation. For example, if Fernando Rodney enters a game in the ninth inning with a man on base, nobody out and a one run lead, every at bat of the inning has the potential to drastically change the game's final result. On the the hand, if Zach Miner is used in the eight inning of game where the Tigers lead 10-0, his performance will likely not have much effect on the outcome of the game.
The Win Probabilty Added (WPA) statistic weights each at bat against a pitcher according to the importance of the situation. In the above example, Rodney's inning would carry more weight in the calculation of WPA than Miner's inning. For more information on the calculation of WPA, see this earlier post.
As a team, the Tigers relievers have a 0.89 WPA (according to FanGraphs) which places them ninth among American League relief staffs. However, if we look at their WPA month by month, we can see that they have improved as the season has progressed:
Additionally, their 2.00 WPA in the past 30 days is second only to the Yankees. So, the Tigers relief staff has been very strong as of late.
Individually, Fernando Rodney's 2.71 WPA for the season is third best in the league behind Jonathan Papelbon (3.70) and Mariano Rivera (3.20). The WPAs for other currently Tigers relievers are listed below:
Fernando Rodney 2.71
Ryan Perry 0.47
Fu-Te Ni 0.29
Bobby Seay 0.29
Brandon Lyon 0.27
Zach Miner -0.01
Rodney has maintained a high WPA all season but others have improved in recent weeks. Brandon Lyon's 1.09 WPA over the last 30 games is second in the league to Joakim Soria (1.18). Ryan Perry is 13th in the league for the same period with a 0.64 WPA.
In conclusion, the Tigers relief staff has gone from one of the league's worst early in the season to one of the best over the past month. Much of the credit for that improvement should be given to pitchers who have been setting up Rodney.