PBR - Questioning Charlie Manuel's in-game decision making is a pastime of sorts for many Phillies fans and sport-talk pundits, but there actually is a metric to quantify the value of a managerial decision or important play.
Win Probability is an advanced metric that can either strengthen or debunk a given argument about how a managerial decision influences a ballgame. The metric can also mathematically identify specific plays in a game and place weighted values on those that significantly impact the final outcome.
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Win Probability fluctuates with every play and is highly dependent on the score, inning, number of outs and placement of base runners. Several matrices exist but a quick and easy one to use can be found on Greg Stoll's Web page [link].
Here's an example: Bottom of the eighth, score tied, runner on first, no one out. The home team has a 70% chance of winning according to Stoll's Win Probability calculator. Let's say the batter drops a sacrifice bunt to advance the runner to second. Turns out the Win Probability actually decreases to 69%, so the sacrifice was a neutral event.
To demonstrate Win Probability I went ahead and charted Sunday's Phillies Nationals game. The graph below shows how the probability of a Phillies victory fluctuated throughout the contest. I went ahead and marked the two events that significantly enhanced the Phillies' odds of winning.
Taking Win Probability a step further, another metric exists called Win Probability Added (WPA). This metric is calculated for every player in a given game and assigns a numerical value to how their contributions helped or hurt his team.
Say, for example, a player comes to the plate in the top of the sixth with his team trailing by a pair of runs with one out and runners on second and third. Putting this situation into Stoll's calculator we can see the visiting team has a Win Probability of 0.369 (37%). If the player at bat hits a single the game situation changes - the deficit is trimmed to one run and runners are now at first and third, so the Win Probability has increased to 0.396 (40%).
Keeping track of WPA, you credit the batter +.027 and debit the pitcher (fielder if an error is committed) -.027. If calculated for every play in a game the total WPA for the winning team should be .5 and the losing team -.5. You can then see how various players performed and see who did the most to help/hurt his club.
Win Probability and Win Probability Added are metrics I often refer to when analyzing player performance and managerial strategy. Moving forward, these metrics will become more intertwined in our articles.
- Patrick Gordon is the editor of the Philadelphia Baseball Review. Contact him at pgordon@philadelphiabaseballreview.com or @Philabaseball on Twitter.