On WPA; Or, why Johnny Damon is your 2009 AL MVP

On WPA; Or, why Johnny Damon is your 2009 AL MVP

Posted by Nick Kapur on Wednesday September 09th 2009, 3:34 am, filed in Diamond cuts

WPA is the stat that old baseball men have always wanted.

If you believe in Wins and RBIs, if you believe that certain pitchers can “pitch to the score” and certain batters are simply “clutch” and can suddenly step up their game when it’s all on the line while somehow sucking the rest of the time, than WPA is the stat for you.

The problem that so many people have with stats like Wins and RBI is not so much with what they show, as indeed, they only show what they were designed to show, and do so quite well. Rather, the problem people have is with what other people think these stats show, which is actually pretty far from what they tell us. Namely, which player really did the most to help his team win games.

But if your goal is to find out exactly which player did the most to help his team win games, than WPA is the perfect stat for you.

Actually, it couldn’t be more elegant. WPA, which stands for “Win Probability Added,” is derived by looking at the current game situation – what inning it is, what the score is, how many outs there are, and how many men are on base – and determining what percentage chance each team has of winning the ballgame in that situation, based on a computer crunching the numbers of all previous baseball games for which complete information is available.

In any given game, the two teams each have about a 50 percent chance of winning at the start of the game (slightly more for the home team, and slightly less for the visitors). With each outcome, whether an out or a base safely reached, one team’s chance of winning increases slightly, and the other’s decreases by the exact same amount, always adding up to 100 percent.

What WPA does, is it awards each batter and each pitcher a certain fraction of a win for each outcome they are involved in, in every game they play in. For example, if a batter gets a single and it increases his teams chance of winning by 2 percent, he is awarded 0.02 of WPA, and the pitcher is docked negative 0.02 of a win. Naturally, getting big hits in crucial situations, or (for a pitcher) getting tough outs with the game on the line, is worth much more of a win than getting hits or outs in blowouts. Over the course of the season, all players’ totals are added to determin exactly how many wins they were actually worth to their team that season. That’s WPA.

WPA differs from WAR in that WAR attempts to assess the overall value a player has provided without reference to game situation. Basically, WAR assumes that a player has little or control over exactly when he gets hits or outs, and thus attempts to assess true skill level, while factoring out random luck. WPA, on the other hand, doesn’t care about true skill level at all. It only cares about how much a player actually helped his team win, based on context. In other words, how “clutch” players were.

Which is why WPA is so perfect for the old school writers and baseball men. Because it is measuring *exactly* what people always thought they were trying to measure with Wins and RBI: how much you helped your team win.

So if we think about the traditional main criteria used by old-school baseball writers to award the two major awards, MVP and Cy Young, which are of course Wins (followed, to a lesser extent, by ERA) for pitchers and RBI for batters (followed, to a lesser extent, by homers), we see that WPA is actually the best way to determine these awards, if these people actually want to measure what they say they want to measure.

Because after all, if you are a crusty old baseball writer, you don’t really care that 50 homers in 1996 was not indicative of Brady Anderson’s “true” skill level – you just know he had a heck of a year.

So, looking at the WPA leaderboards for each league in this particular season (minimum 4.00 WPA), we get the following:

National League
Albert Pujols – 7.22
Prince Fielder – 7.06
Ryan Howard – 5.38
Chris Carpenter – 5.08
Joey Votto – 4.76
Chase Utley – 4.43
Tim Lincecum – 4.27
Andre Ethier – 4.04INKE #1,

American League
Zack Greinke – 4.81
Justin Verlander – 4.31
Johnny Damon – 4.27
Jason Bay – 4.04

No big surprises in the National League, where if you went by WPA, you’d wind up with Albert Pujols as MVP and Chris Carpenter as Cy Young, both of those players being the odds-on favorites to win if the season were to end today.

But the American League is a different story, as WPA shows how Wins and RBI fail to tell the whole story of who happened to be the most clutch. Zack Greinke is first overall in WPA, despite being behind the pack in Wins, and Johnny Damon is your leader among position players in WPA.

Indeed, WPA suggests that Zack Greinke was actually the most valuable player in the American League in this particular season, and probably should be the MVP, but if we adhere to the traditional rules that pitchers should not be MVPs and the MVP has to come from a playoff-bound team, than by all rights Johnny Damon ought to be your American League MVP frontrunner.

But in any case, Zack Greinke is far and away your AL Cy Young this year. Not only is he leading the league in WPA, showing his “clutchness,” but he’s also leading all AL players in WAR, showing his “true” skill level!

Interesting method to look at player value. Verlander showing up as #2 in the AL is great. If we have Damon also, we may have a chance at winning the AL Championship this year.