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Win Probability Added (WPA) is a stat that attempts to measure how much impact each play had on the outcome of a game and assigns that value to the players responsible. In other words, hitting a grand slam in the seventh inning when the score is 10-2 has considerably less WPA value than drawing a walk to lead off the ninth inning when the score is 2-2. The grand slam didn't have much impact on the likely outcome of the game, whereas the walk had a major impact on each team's chances of winning.

There are much better and longer explanations of WPA than that one, of course. If you're interested in learning more about it, Dave Studeman's WPA primer at The Hardball Times is a good place to start, and both Fan Graphs and Wikipedia offer tons of good information on the subject. It's far from a perfect stat and is not meant to definitively prove how valuable each player has been, but WPA is an interesting tool to use in looking back at what has already taken place.

It's important to note than WPA doesn't measure any defensive contributions, which means that strong defenders don't receive full credit for their value. Beyond that, WPA doesn't place offensive contributions in the context of position, so an .850 OPS from a catcher or shortstop is treated the same as an .850 OPS from a designated hitter or left fielder. There's nothing that can be done about measuring defense via WPA, but it's relatively easy to put the numbers in better context by using positional adjustments.

With the help of David Gassko from The Hardball Times, I've taken the overall WPA totals found at Fan Graphs and adjusted them for position. Many of the adjustments are minimal, but starting pitchers are given a boost relative to relief pitchers and hitters who play up-the-middle positions are given a boost relative to hitters who man corner spots. The end result is a sort of adjusted WPA (adjWPA), which you can find below:

                      adjWPA                               adjWPA
Johan Santana          2.62          Boof Bonser           -0.13
Pat Neshek             2.22          Glen Perkins          -0.16
Joe Nathan             1.77          Jason Bartlett        -0.35
Justin Morneau         1.40          Kevin Slowey          -0.36
Joe Mauer              1.35          Lew Ford              -0.40
Luis Castillo          0.93          Dennys Reyes          -0.45
Carlos Silva           0.74          Jeff Cirillo          -0.49
Matt Guerrier          0.63          Ramon Ortiz           -0.74
Torii Hunter           0.45          Luis Rodriguez        -0.82
Michael Cuddyer        0.24          Jason Tyner           -0.88
Mike Redmond           0.21          Sidney Ponson         -0.92
Scott Baker            0.20          Nick Punto            -1.07
Juan Rincon            0.01          Jason Kubel           -1.85

Between his Babe Ruth-like heroics at the plate and a 2.60 ERA in 128 innings, Johan Santana is the clear-cut adjWPA leader through 92 games. Seeing Pat Neshek's name in the second spot might be surprising, but it probably shouldn't be. Not only does Neshek have a 1.59 ERA and .127 opponent's batting average in 45.1 high-leverage innings, he's put out tons of fires for both starters and relievers by stranding 87 percent of the baserunners he's inherited. That's the type of thing that WPA accounts for.

Joe Nathan has been more hittable than last season, but has a 2.03 ERA while throwing 40 innings in almost exclusively high-leverage situations and has converted 18-of-20 save chances. Justin Morneau holds a huge lead over the rest of the position players in raw WPA, but Joe Mauer closes almost all of that gap thanks to the difference between first basemen and catchers. In other words, a .312/.405/.455 hitting line with 40 RBIs and 44 runs scored in 60 games from a catcher is pretty damn valuable.

Mauer ranking fifth in adjWPA is pretty amazing given that he's missed one-third of the games and his value would be even higher than that if defensive contributions were included. Also amazing is that two-thirds of Luis Castillo's sixth-ranked adjWPA total comes from the recently completed four-game series against the A's. Castillo had a .31 adjWPA coming into the series, but racked up a .62 adjWPA in four games against Oakland. Like Mauer, his value would rise even further with defense factored in.

Torii Hunter's adjWPA pales in comparison to his overall numbers, which was also the case last season. He's hit just .229/.264/.271 in close-and-late situations, which is among the worst production in the league in those spots, but has batted .333/.370/.549 when the margin is at least four runs either way. WPA reflects the fact that he's struggled in high-leverage situations while piling up a large portion of his impressive totals in spots where the outcome wasn't necessarily in doubt.

Of course, a .45 adjWPA is still plenty good. It ranks seventh among MLB center fielders, whereas Hunter ranks fifth among MLB center fielders in VORP, which treats all hits equal regardless of impact. By comparison, Mauer and Morneau both rank fourth in adjWPA at their position, while Castillo ranks 12th among MLB second basemen. Similar to Hunter, Michael Cuddyer perhaps ranks lower than expected thanks to mediocre numbers in high-leverage spots and a team-high 11 double plays.

Switching to the bottom of the rankings, adjWPA shows Jason Kubel as the clear least-valuable player. Not only has he hit just .241/.294/.390 overall, which is horrible for a corner outfielder, Kubel has batted .200/.238/.375 in close-and-late situations and .200/.231/.400 with two outs and runners in scoring position. Nick Punto, Sidney Ponson, Jason Tyner, Luis Rodriguez, and Ramon Ortiz round out the rest of the least-valuable group, which should come as no surprise.