Win Probability Added (WPA) measures a baseball player's contribution to the probability that his team wins a given game. So it's a situational stat that takes into account only the context and the result. If you strike out in the first inning, you might decrease your team's chance of winning by 2 percentage points, and therefore get a WPA of -0.02 for that event. If you come up to bat in the bottom of the 9th with 2 outs, behind by 1 run, and hit a 2-run walkoff HR, you might increase your team's chances of winning from, say, 30% to 100%, and you'd get a WPA of 0.70 for that event. Similarly, if the pitcher strikes you out to end the game in that plate appearance, he would get a WPA of 0.30.
So here's the trivia question: What 2 Red Sox players led the team in WPA in games 4 and 5, respectively, of the 2004 ALCS?
Your only hint: The answer is NOT David Ortiz, for either game. In each game, Ortiz finished 2nd on the Red Sox in WPA.