I Think Win Probability Added Is a Neat Statistic

We’re in a tiny lull in the baseball season, and honestly, I’m happy about it. July is jam packed with draft and trade talk, September and October are for the stretch run and the postseason, but the middle of August is when everyone catches their breath. There’s no divisional race poised on a razor’s edge, no nightly drama that everyone in baseball tunes in for; it’s just a good few weeks to get your energy back and relax.
For me, that means getting a head start on some things I won’t have time to do in September, and there’s one article in particular that I always want to write but never get around to. I’m not a BBWAA member, and I’ll probably never vote for MVP awards, but I spend a lot of time thinking about them every year nonetheless. When I’m looking at who would get my vote, I take Win Probability Added into account. Every time I mention it, however, there’s an issue to tackle. Plenty of readers and analysts think of WPA as “just a storytelling statistic” and don’t like using it as a measure of player value. So today, I’m going to explain why I think it has merit.
First, a quick refresher: Win Probability Added is a straightforward statistic. After every plate appearance, WPA looks at the change in a team’s chances of winning the game. We use our win expectancy measure, which takes historical data to see how often teams win from a given position, to assign each team a chance of winning after every discrete event. Then the pitcher and hitter involved in that plate appearance get credited (or debited, depending) for the change in their team’s chances of winning the game. Since every game starts with each team 50% likely to win and ends with one team winning, the credit for each win (and blame for each loss) gets apportioned out as the game unfolds. The winning team will always produce an aggregate of 0.5 WPA, and the losing team will always produce -0.5, spread out among all of their players. Read the rest of this entry »