Maybe it's time to start another WPA thread. We need something to liven things up here. PM is the resident guru of WPA, but he does have his detractors.
Yes, WPA can be positive or negative. It tries to show which players contributed most to positively winning a game or negatively losing it. In the game last night Felix would have had a positive score, but League a very negative one. The sum total of all WPA for the Mariners would be minus one, and the sum total for the Inidans would be plus one for the game. Am I right Pirata?
Pretty much Vidya, except for the part that you start the game with a 0.5 chance of winning it, so you only need to "add" another 0.5 to win it. Remember that probabilities oscillate between 0 and 1, so if your probability of winning the game is 0, it means that it is CERTAIN that you will lose it, and if it is 1 then it is CERTAIN that you will win it. This only happens when the game ends. At the beginning of the game, you start off with 0.5 (50%) chance of winning it, and from then on, each play "adds" or "removes" probability of winning it. If you do something to help your team you add probability, hence your WPA will be positive, if you do something that hurts your team (like League yesterday), then you "remove" probability, hence your WPA will be negative.
Take for instance yesterday's game:
Félix: 0.522 (more than 50%)
League: -.731 (yes, negative .731)
you add them all = -.176 pitching
Figgins: -.073 (hurt the team)
Ackley: .09 (helped)
Total hitting: -.326
Total WPA = Pitching WPA + Hitting WPA = -.176 + (-.326) = -.5
So, you can see that the hitting (no kidding) was the main culprit of losing yesterday's game.