Rodney Better Than His ERA

Reliever performance is always a topic of debate among baseball fans. Just looking at the Tigers this year, there is a whole array of opinions on Fernando Rodney. Some think he has been great because he has not blown an official save opportunity this year. Others say he is not very good because he pitches poorly in situations when a save is not on the line and allows too many base runners in general. Statistical evaluation of relievers is difficult for a couple of reasons:

1. They pitch so few innings that their statistics can be influenced heavily by a couple of really bad outings.
2. Their actual value depends on game situations more than any other player.

Using ERA to evaluate relievers is problematic because relievers often come in with runners on base and give up other pitcher's runs. So a pitcher could have a low ERA without actually being that effective. FIP ERA which is based on walks, strikeouts and home runs allowed rather than runs allowed is better but it still does not consider the game environments in which a reliever pitched.

In one instance, Brandon Lyon comes into the game with a one run lead and two runners on base in the 8th inning. In another game, Bobby Seay comes into the game with a 6 run lead and nobody on base. Now suppose each pitches a perfect inning. Using ERA or FIP, they would both get the same credit for that inning but Lyon's performance had more impact on the outcome of the game.

The Win Probability Added (WPA) statistic gives players (hitters, starters, relievers) credit based on the effect each player has on his team's probability of winning. These probabilities vary depending on the game score, the runners on base and the number of outs before and after each play. They are based on the results of thousands of games worth of data looking at every possible situation over and over.

More concretely, WPA works as follows. Suppose Bobby Seay comes into the game in the top of the 8th with a 2 run lead, 0 outs and a runner on first. According to the Win Expectancy Finder at Walkoffbalk.com, there is .787 (78.7%) expectancy that a team will win the game given that situation. Suppose Seay strikes out the first batter. There is now one out and the probability of winning has gone up to .848. Thus, the strikeout was worth .848-.787=.061

Now suppose the next batter after that doubles home a run. The Tigers now have a one run lead with a runner on second and one out. The probability of winning goes down to .693. So Seay loses points on that batter: .693-.848=-.155.

If you add up all the gains and losses for all the batters Seay faces you get his WPA. WPA doesn't necessarily solve the problem of small sample sizes but it is a reasonable stat for relievers because it gives more weight to plate appearances which have a strong impact on winning and losing games.

As a team, the Tigers bullpen has a WPA of -4.35 which places them 11th in the American League. If we had used ERA or FIP instead, they would have fallen even further down the list. So, the performance of Tigers has been below average this year. Table 1 below lists The WPAs for Tigers most frequently used relievers in 2009. Table 2 lists the American League leaders. These data were abstracted from the Fan Graphs web site.

Table 1: WPA for Tigers Relievers

Pitcher	G	WPA
Fernando Rodney	38	1.75
Bobby Seay	38	0.30
Ryan Perry	26	-0.17
Zach Miner	19	-0.18
Nate Robertson	21	-0.19
Joel Zumaya	27	-0.78
Brandon Lyon	34	-0.93

Table 2: American League WPA Leaders

Pitcher	G	WPA
Jonathan Papelbon	38	2.54
David Ardsma	40	2.18
Mariano Rivera	36	1.92
Frank Francisco	27	1.91
Fernando Rodney	38	1.75
Joe Nathan	35	1.71
Jim Johnson	35	1.59
Darren O’Day	31	1.58

Fernando Rodney is the leading Tigers reliever according to WPA (1.75) so far this year. This ranks Rodney 5th in the American League. If we ranked Fernando according to his by ERA (4.03), WHIP (1.32) or FIP (3.97), he would not have finished in the top 30. He ranks better on WPA because, like most closers, Rodney comes into a lot of critical situations and gets positive results a lot more often than negative results. He has not pitched as well in less critical situations (like a four run lead in the ninth inning) but those games do not carry as much weight with this statistic. The only other Tigers reliever with a positive WPA is Bobby Seay at 0.30.