You might be asking yourself, "what are these charts and graphs that keep getting posted every day?" I am going to try to give you a simplified description and please feel free to ask any questions.
WPA stands for Win Probability Added. Over the last few years, several people have done research on the effect each event, within a baseball game, has on the final score. Many data tables have been created to analyze thousands of games in order to determine each situation's impact on the team's eventual win or loss. At the start of each game, each team has a 50% chance of winning that game. If the leadoff hitter begins the game with a home run, there is a definite shift in that percentage toward the visiting team (actually 9.9%). With the score now 1-0, 0 outs, and no one on base, the visiting team should win the game 59.9% of the time. This is what is known as the Win Expectancy of each team.
Now, for what the term "Win Probability Added" means. In the above example, the hitter receives credit for the 9.9%, while the opposing pitcher receives a decrease of 9.9% for the event. You are giving credit to the player(s) responsible. You can also give credit to a fielder for his effort in an event.
Each subsequent event in the game has a similar impact. A home run, fly out, walk, wild pitch, error, etc. increases/decreases the expectancy that each team will eventually win the game. The impact level can change the team's Win Expectancy. That level depends on the score, inning, number of outs, and runners on which base(s). Each event is has a level of impact at that point in the game. You can not go back and reanalyze events after the game is over.
To complete the chart of players, all the events that involve each player are tabulated. This is their total WPA for the game. If you've done it right, the total for each team will either be +50% or -50% since both teams started the game at 50%.
The chart I post after each game shows the "narrative" of the game. It displays in graphic form the increases and decreases in Win Expectancy. Were the Brewers really in control the entire game, how much of an impact did that home run have, etc.
WPA should not be used to identify clutch hitting or seasonal totals. It only takes into account what happened in a single game. It does not give a true analysis of a player's skill because the player has no control over the situation in which he finds himself. Those events are random and there are other statistics that can be used to identify a player's ability over the long term.
What I have found interesting is that in the majority of the time, a sacrifice bunt has a negative impact on the team's Win Expectancy. Why? While you may be advancing a base runner, you are subtracting an out in the inning and depending on the situation, that can have a huge impact. The timing of managerial moves can be second guessed (bunt, steal, pitching changes, pinch hitters, etc.). You can also determine if having a pitcher strikeout while batting with the bases loaded is better than hitting away.
Dave Studeman, has written many articles for The Hardball Times and has created the spreadsheet I have been using. In the spreadsheet, Dave has integrated the run environment. This affects the Win Expectancy based on the home park. He really has taken the concept to the next level through the spreadsheet. Also, the Win Expectancy is based on mathematical calculations. Therefore, the spreadsheet is always current and up-to-date. I have used some of his information in my post and if you would like to read more about Win Probability Added and Win Expectancy, please read his two articles below: