I can only imagine how Twins fans felt when they learned this morning that Joe Nathan will likely miss the 2010 season. Not only will they have to place a less reliable reliever in the closer’s role, but they’ll have to replace the last man in their bullpen with an even lesser reliever. This certainly changes their season outlook. It might even end up costing them prospects if they eventually determine that they need to bring in a closer from another organization. They’re still the favorites in the AL Central, but the gap just got a bit narrower.

Since his trade to Minnesota and move to the closer’s role, Nathan has pitched six magnificent seasons. his highest ERA in that span was 2.70, which came five years ago. He has converted 247 of 272 save opportunities in that span while posting a 0.93 WHIP, 11.1 K/9, and a 236 ERA+. There’s no doubt he ranks among the best closers in baseball of the past decade. In fact, there might be only one who ranks ahead of him. That, of course, is our own Mariano Rivera.

Photo credit: Kathy Willens/AP

Over the past decade, only 24 pitchers have pitched 500 games in relief (I’ll count Nathan, who is the 24th and has 499). Mariano ranks third with 651, but ranks second in innings pitched at 713.1, just behind David Weathers. Yet he has walked the third fewest batters, 137, despite pitching 171 more innings than Keith Foulke (130) and 180 more than Trevor Hoffman (124). He has the lowest ERA, by 0.32 points. He has also pitched 133 more innings than the next lowest pitcher, Billy Wagner. Mo’s 38 home runs allowed ranks second in this group. The pitcher ahead of him, Mike Stanton, pitched almost 200 fewer innings.

The reason Mo has been able to post such brilliant performances is that he’s stayed healthy. In only one season since 1996 has he appeared fewer than 61 times. That’s 13 healthy seasons out of 14 as a Yankee regular. How many other closers come even close to that? Even Trevor Hoffman has fallen below the 60-appearance mark four times since 1996, including both of the last two seasons. The only other current closer who comes close is Francisco Rodriguez, who has hit the 60-appearance threshold in ever season since 2003 (he had 59 that season, but close enough).

Closers come and closers go. Constant through all of it is Mariano Rivera. We don’t need a special occasion to gush about his greatness, but when situations like Nathan’s do arise it allows us to truly appreciate Mo. He is without peer.

Nine years ago, Voros McCracken presented his theory of Defense Independent Pitching, or DIPS. In a well-researched bit for Baseball Prospectus, he showed that while a pitcher’s strikeout rate, walk rate, and, to a lesser extent, home run rate remain fairly consistent from year to year, the number of hits they allow on balls in play does not. Using this data, he concluded that, “There is little if any difference among major-league pitchers in their ability to prevent hits on balls hit in the field of play.” This ran counter to conventional thinking, but he brought enough data to the table that his findings were tough to refute.

Fielding Independent Pitching, or FIP, was borne of DIPS. Consistent with McCracken’s theory, FIP considers only a pitcher’s home run, walk, and strikeout rate. This isn’t to say that all balls in play are created equal. Instead, it is to say that balls in play are subject not only to the pitcher, but to the hitter at the plate and the defense behind the pitcher. With those three factors at play it becomes difficult to assign blame. FIP simple views a pitcher through the lens of what he himself controls.

Yet we know that a pitcher does have a degree of control over batted ball type. Some pitchers keep the ball on the ground, like Brandon Webb, while others induce more fly balls, like Jered Weaver. A pitcher’s batted ball tendencies add another dimension to analysis. Recently, Matt Swartz and Eric Seidman of Baseball Prospectus introduced their solution: SIERA, which stands for Skill-Interactive Earned Run Average. It attempts to combine batted ball types, in addition to strikeout, walk, and home run rates, in order to get a better gauge of a pitcher’s true skill.

While the five-part explanation resides behind Baseball Prospectus’s pay wall, they have been fairly up front with the formula. It might seem complicated, but it essentially takes the various factor and runs them through adjustments in order to not only put a value on a pitcher, but scale it to ERA. Peter Hjort of Capitol Avenue Club was kind enough to create a SIERA calculator, complete with pitchers from 2009. But that’s not what I’m interested. What gets me thinking more is how SIERA views a pitcher who has been consistently underrated by FIP.

In the comments of Rebecca’s post about Mariano Rivera’s playoff WAR, commenter CB noted that FIP does not treat Rivera fairly. It consistently estimates his ERA above his true level, and that has become more and more true in recent years. Sure, Mo’s FIP is consistently in the low- to mid-2s, because he doesn’t allow a lot of home runs, doesn’t walk batters, and usually strikes out more than a batter per inning. My question is, does SIERA provide a better look at Mo?

We’ll look at the years 2002 through 2009, since that covers the FanGraphs era.

Until 2007, FIP came much closer to Mo’s ERA than SIERA. His ERA skyrocketed that year, mostly because of a horrendous April (10.57 ERA), so SIERA, which for Mo was consistently higher than FIP from 2002 through 2007, came closer. In 2008 both formulas came to the same conclusion, and finally in 2009 SIERA predicted a lower ERA than FIP. Problem is, neither came all that close. Once again, Mo performed better than any fielding independent metric could have predicted.

(For those wondering, that counts tERA, though it came damn close in 2008.)

Despite the discrepancy, I do not consider this a bug in SIERA. In fact, I consider it a feature. I mean, if a projection system can accurately project god, well, then where do we have go go from there?

Credit: AP Photo/Rob Carr