For those of you who follow fangraphs and sabermetrics pretty extensively, you know what WPA is. For those of who you do not, WPA stands for Win Probability Added. Let me try to explain this in extreme layman's terms. During the start of a baseball game, the exact odds one of the teams will win the game is 50%. I don't care if it's the Pittsburgh Pirates facing the New York Yankees, at the start of the game, the Pirates have a 50% change of winning that individual game. But throughout the course of the game, the odds a particular team wins the game changes. If Derek Jeter is the first batter of the game and he hits a home run, the odds of the Yankees winning the game slightly improve. There are still 27 outs for each team to work with so the Yankees don't get a huge edge at that point in the game, but their odds do increase and the score is becomes 1-0.
However, if the score is tied, it's the top of the 9th, and Derek Jeter hits a home run to lead off the inning, well the odds that the Yankees will win the game increase dramatically. In both instances the Pirates are down by one run but the odds they can come back to win the game are different because of the amount of outs they have to work with.
The odds a team wins the game is how to determine WPA. According to Fangraphs, the definition of WPA is
the difference in win expectancy (WE) between the start of the play and the end of the play.And how each player does to affect the odds of his team winning the game helps goes into determine his indivudal WPA. According to Fangraphs:
That difference is then credited/debited to the batter and the pitcher. Over the course of the season, each players’ WPA for individual plays is added up to get his season total WPA.Here's an example Fangraphs gives to help quantify this:
In Game 4 of the 2007 World Series, the WE for the Rockies started out at 50%. When Jacoby Ellsbury doubled off Aaron Cook in the very first at-bat in the game, the Rockies WE declined to 44.2%. The difference or WPA was .058 wins (5.8%). Ellsbury was credited +.058 wins and Aaron Cook credited with -.058 wins.This same exact approach can also be applied to football.
In 1988, Bob Carroll, John Thorn, and Pete Palmer became the Bill James of football. That year they published and released the book The Hidden Game Of Football which was the world's first introduction to advanced statistics for football. This book sets the foundation for websites like Football Outsiders and Advanced NFL Statistics.
In the book, they talk about the odds a team most likely is to win a game. In baseball, all you have is 27 outs per team to score as many runs as possible. Therefore, the three main factors to determine WPA are: how many outs are there left in the game, how did you get on base/ who is already on base, and what the score of the game is. However, in football, you not only have to deal with the clock, but you also have to deal with downs. Therefore, there are five main factors that goes into football WPA: what the score of the game is, how many downs are left, how many yards did the last play gain, where you are on the field, and how much time is there left to play.
In baseball, a home run to lead off the game has a far less impact on the WPA of the game that a home run to break a tie in the 9th inning. In football, a 20 yard pass to start the game has a far less impact that a 20 yard pass to start your last drive on the game in a tie. Along similar lines, a three yard rush on 1st and ten has a far less impact on a team's WPA than a three yard rush on 3rd and two. Along more similar lines, the aforementioned play has less of an impact than a three yard rush on the second to last play of the game allowing your kicker to kick a 47 yard field goal as opposed to a 50 yard field goal to win the game.
The Hidden Game Of Football laid out in detail the odds of scoring points. Plus, basic logic says it's a lot harder to drive 99 yards down field to score a TD than 49 yards. In fact, it's this logic that's it's not necessarily a bad idea to go for a TD on 4th and 1. Because if you don't convert the 4th down to score, you still have a pretty good chance to score again due to the fact that it's EXTREMELY hard for your opposing team to score so most likely you have have great field possession on your next drive to score again. For anyone that saw Week One Bears vs. Lions, this happened. Matt Forte and the Bears failed to convert 4th and 1 so the Lions got possession of the ball on their own one yard line. They failed to make a first down to the Bears got great field possession on their next possession. In fact this happened for like three straight drives- this back and forth. (I'm sure this has happened at least once to every NFL franchise and/or it's opponent)
Now that I have briefly explained what WPA is, I will now show you what I think is the greatest football game I have ever seen: and that was the Week Six game between the Chicago Bears and Arizona Cardinals in 2006. This also led to my favorite post season rant of all time. "The Bears are who we thought they were!". Here is the WPA for that game:
At halftime, the Arizona Cardinals had essentially locked this game up. With 5:53 left to play, the Cardinals were winning 23-10. At this point, the Cardinals had a 99.97% of winning this game. Then, 42 seconds later, Brian Urlacher recovers an Edgrerrin James fumble and scores a touchdown. However, at this point, the Cardinals still had a 66% chance of winning this game (as the score was still 23-17 in Arizona's favor). Then with 3:17 left to play, Devin Hester returns a punt for a touchdown giving the Bears their first lead of the game 24-23. Within a matter of about two minutes, the Cardinals go from a 66% chance of winning down to a 66% chance of losing. But Arizona got the ball back and marched down field. At the 1:04 mark, that 66% chance of losing rose to 78% chance of winning. Even if Arizona doesn't get a touchdown, they still get an easy field goal to win the game 26-24. But as Neil Rackers kicked a 40 yard field goal with 52 seconds left to play, the ball went wide left and the field goal attempt was no good. And with that kick, Arizona's 78% chance of winning the game went to a 2% chance of the winning the game. And 52 seconds and a few kneel downs later, that 2% diminished to 0%.
Now those two numbers in the bottom right corner indicate the's game Excitement Rating (out of 10.0) and Comeback Rating (out of 100). According to the WPA, the excitement rating was just below average, but the comeback rating was perfect. So you can make the argument that this wasn't the greatest game ever, but just the greatest comeback. Sure, you can make that argument.
Source of WPA and a the WPA chart: Advanced NFL Stats
Now I hope this post also gave you a great introduction into WPA- for both players and teams. In baseball, we don't use WPA to determine who the best player is, we use a great stat called WAR- which takes into account the quality of a player's offense and defense and compounds it into a nice round number for us. But football does not have that statistic and it probably never will. As Bill Bramwell, co-author of Football Outsiders, says on the Freakonomics blog:
We’re nowhere near the point of valuing a player as being worth a number of wins, because we’re light-years away from quantifying all the things a player does. I doubt we’ll ever have a reliable “wins” metric because there are too many interactions between positions that we can’t account for in football. Take a quarterback, for example: even if we were to develop a measure of performance that stripped out the effects of his receivers and offensive line and placed his passing performance in a perfect, league-average context, we’d have to account for how he read defenses and called audibles at the line, how effective he was in setting up defenses on the play-fake, whether he had any impact on the running game versus an average quarterback … it’s not a realistic goal.WAR can help determine exactly how many wins a particular player contributes to his team in baseball. But because of the dependent nature of football, we will probably never get to that point. However, WPA comes really close. Although we can't account for the audible a QB makes to switch from a pass play to a run play so his RB can gain ten yards, we can account for actual results that occur.
And let's face it, all we want are results. We want to see our team's offense drive down the field to score points and we want to see out team's defense stop the opponent from scoring points. We don't really care how that result occurs (although we love to argue about it) as long as it does. And that's why I think WPA is a great step in the right direction to help isolate how players have performed.
But like I said, WPA is not the end all, be all. Advanced NFL Stats also has a stat called EPA (Expected Points Added) and Football Outsiders have some great stats called DVOA and DYAR. I also don't think this has a very good predictive element at all so I wouldn't use it for fantasy purposes or for predicting statistics the year after. But I think they are great statistics for helping you explain the past.