Philly's Inferno

A sports fan’s opinion through the lens of Philadelphia’s seventh circle of hell.

Month: May, 2013

What Contributes to Home Field Advantage in the NFL?

Note: If you would like to jump to the interactive NFL Home Field Advantage infographic, click here.

I will never forget “Fourth and 26”. It was early in the morning when we arrived. The parking lots at Lincoln Financial Field were already filling up and it was brutally cold. My friends and I dressed for it – I wore twelve layers – and we brought plenty of food to grill and plenty of beer to drink. The beer we had to drink quickly, because at eight degrees a layer of ice formed on the surface. More than once I tilted a half- full can to take a sip and nothing followed. It was a cruel joke. Regardless, the twelve layers and amount of beer I drank made taking a leak a torturous endeavor. But in reality, there was no better place I would rather have been than that frozen hell. All because of Freddie’s hands. God bless them.

After Donovan McNabb delivered a hard-thrown wobbler that Freddie Mitchell stopped and secured for that critical first down, the Linc erupted. It’s a sound I will never forget. It was a deafening roar. My buddy and I screamed in disbelief and hugged everyone within, and just out of, reach. The Eagles were losing to Green Bay by three points at the time, and that miracle play gave the Eagles a new set of downs, which McNabb used to get into field goal range. There was not a doubt in my mind that David Akers was going to kick the 37 yard field goal needed to force overtime, and the Eagles were going to win it.

There were other great plays in that game (like Brian Dawkins’ interception of Brett Favre in overtime), but would these plays have happened if the game were played in Green Bay instead? Does this game illustrate the importance of home field advantage? It’s actually a question I didn’t intend to ask. This post was initially going to be about whether Chip Kelly’s offensive success formula can be applied to the NFL. In short, it does. But during my research I noticed that home teams have a decided advantage, so my attention shifted.

REVIEW

Home advantage in sports has been a popular research topic. A quick comparison of home team wins to away team wins will demonstrate that the phenomenon exists. For example, in their 2011 book Scorecasting, economist Toby Moscowitz and Sports Illustrated journalist Jon Wertheim looked at NFL wins from 1966 through 2010 and concluded that home teams win 57.3% of the time. The bigger question, and one they address, is why does home field advantage exist at all?

According to Freakonomics.com writer Stephen J. Dubner, Moscowitz and Wertheim discounted, through research, many of the popular reasons we think home field is an actual advantage:

When athletes are at home, they don’t seem to hit or pitch better in baseball … or pass better in football. The crowd doesn’t appear to be helping the home team or harming the visitors. We checked ‘the vicissitudes of travel’ off the list. And although scheduling bias against the road team explains some of the home-field advantage, particularly in college sports, it’s irrelevant in many sports.

Instead, Moscowitz and Wertheim adopt a theory proposed by a German economist, Thomas J. Dohmen. Dohmen addressed how decisions are influenced by social factors that often contradict our own best interest by looking at how decisions by referees in Germany’s Bundesliga are affected by the home crowd. He concluded that it’s not the crowd’s impact on the players that matter, but it’s the referees who are subconsciously affected. Dohmen found that German referees award more extra time during games in which the home team is behind by one goal, and more time when the game is tied but the home team is in a good position to score.

Sabermetrician Phil Birnbaum summarized Moscowitz and Wertheim’s adoption of this theory to the NFL, in terms of penalties and instant replay decisions:

In the NFL, “Home teams receive fewer penalties than away teams — about half a penalty less per game — and are charged with fewer yards per penalty. Of course, this does not necessarily mean officials are biased. But when we looked at more crucial situations in the NFL … we found that the penalty bias [increases].”

Away teams have their challenges upheld 37 percent of the time, versus 35 percent for home teams. But when the home team is losing, the visiting team wins 40 percent, versus only 28 percent for the home team. So it looks like the referees favor the home team more when they need it more.

RESEARCH

I don’t have data to support replay decisions, but on the surface, my own research seems to support Moscowitz and Wertheim’s suggestion that there may be a penalty bias, however subconsciously. From 2009 to 2011, NFL home teams averaged 6.08 penalties (offense plus defense) per game, while away teams averaged slightly more, 6.18. Offensive penalties were nearly identical, with home teams averaging 3.35 and away teams averaging 3.34 per game. The largest difference in penalties appears on the defense, where away teams average 2.84 penalties per game and home teams average 2.73. In addition, NFL home teams won 434 games (56.5%) by an average score of 23.12 to 20.67. Is this enough to explain home field advantage?

To answer this question, I’m using what I call an offensive efficiency rate (OER). This is not to be confused with the Defense-adjusted Value Over Average (DVOA) that FootballOutsiders.com uses to calculate offense efficiency. Instead, my efficiency rate is a bit simpler (if you’re familiar with my work, you may remember that I subscribe to a Gladwellian “less is more” kind of philosophy). I define offensive efficiency as:

(rushes + pass completions) / total offensive plays,

where total offensive plays is the sum of rushes, pass attempts, and offensive penalties.

I calculated OERs for home and away teams for the 768 games in my sample, then performed a logistic regression to determine if OERs could predict game outcomes. The results were very conclusive (p-value <.00001). When the home team has a higher OER, the home team wins 89% of the time. When away teams have a higher OER, away teams win 80% of the time. Since the results were significant, I converted the individual team/game OERs to home and away win probabilities. And this is where I observed something I thought interesting. When both the home and away teams have identical OERs, the home team has a higher win probability. For example, when the OERs are equal to 75%, the home team has a 62.6% chance of winning, while the away team has a 46.2% chance of winning. Is this evidence of home field advantage?

Graph 1 PIHF1

I wasn’t sure so I sought help. I reached out to Phil Birnbaum, the sabermetrician who wrote a great review of Scorecasting and provided some interesting insight into Moscowitz and Wertheim’s findings. He suggested to me that, since home teams win more often, they may rush more often to protect a lead, which would in turn inflate their OERs. Since my data does not include play types per quarter, I randomly removed 100 home wins from my sample so OER averages were weighted the same (334 wins) for both home and away teams. I also decided to control for offensive penalties in order to see if there was still a home field advantage in this “equalized” environment, a relative vacuum. According to these new results, home and away win probabilities are nearly identical:

Graph 2PIHF2

Then, when penalties are included, win probabilities extend slightly in favor of the home team:

Graph 3PIHF3

This graph is interesting because it shows a small away team advantage as OER decreases. However, when including penalties in an equalized environment, there does seem to be a slight home field advantage as OER increases. When both teams have an OER equal to 80%, the home team has an 80.8% chance of winning, while the away team has a 73.4% chance of winning. Since we know what HFA looks like in a controlled, equalized environment, and what it looks like in reality (Graph 1), what about turnovers?

In reality, turnovers are the great equalizer. If the home team has just one less turnover than the away team, then home teams have a decided advantage. When OERs are equal to 75% and away teams have one more turnover than home teams, home teams have a 78.5% chance of winning, while away teams have a 28.5% chance of winning:

Graph 4PIHF4

And when home teams have one more turnover than away teams, home field advantage is all but erased:

Graph 5PIHF5

Of course, there are exceptions. For more team-specific details, below is an infographic on NFL Home Field Advantage. Click on it to interact. I determined HFA by subtracting each team’s average Home Win Probability from their Home Opponent’s Win Probability. The larger the difference, the better the HFA. Using this rubric, the Green Bay Packers had the best home field advantage from 2009-2011, while the Washington Redskins had the worst.

PIHF6

CONCLUSION

Home field advantage definitely exists. As Moscowitz and Wertheim suggest and my own research illustrates, penalties do seem to have a slight influence. However, I cannot claim that this is caused by subconscious referee bias. I also cannot discount the power of the crowd. The fact that away team defenses average more penalties than home team defenses could indicate a referee bias, or it could indicate an away team defensive player’s reaction to a high pressure social situation (the home crowd).

Also, not quantified here, is the home crowd’s impact on the home defense. As Birnbaum suggests, and I agree, humans evolutionarily have a need to defend their home. In football, this is extremely applicable. My guess is, when the home defense is placed in a high pressure, high emotion, high adrenaline situation, they will more often execute its collective desire than the away team’s offense. In other words, even if both team’s may have the same OER, the home defense, feeding off the energy of the crowd, wanting to defend its home, is more likely to obtain that “key, timely stop” than when playing away. Perhaps this is why Darren Sharper did not defend Freddie Mitchell like he was supposed to. So Eagles fans, continue the deafening roar.

Special thanks to Phil Birnbaum. Check out his work at http://blog.philbirnbaum.com and http://www.philbirnbaum.com

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On Family and Football

Intentionally I have not been fully transparent about my personal life when writing, but this post is a little different. Last September my wife Liz gave birth to our first child, our son James. He’s eight months old today and a porker, a chunker, a pooper, a stinker, a laugher, a cuddler, and the most incredibly striking thing I have ever seen. I mention this because I decided when he was three months-old to start writing. As you might imagine Liz was not too thrilled about it. But I guess I have a way with words…

Unknown when I started Philly’s Inferno was the community I was about to enter. There are others out there, those who enjoy writing and are passionate about the subject matter. It’s a sub-culture of Eagles Nation bloggers and tweeters and intelligent social fandom I never knew existed. And damn, they’re good. I’m not sure if my passion has shown through quite as much, or as well, but it’s something for which I will continue to strive.

This post is my 51st since my journey started in December. That number still baffles me. What’s even more baffling is the thousands of times people have read what I wrote. Thousands… I’m so humbled by that. They read my Eagles’ opinions, my statistical analysis, and my simulations. I have learned from them about football, about writing, and writing has taught me much about myself. Thank you everyone, for your comments, your insight… Thank you all for reading. But there are only two readers who really matter.

Liz has supported me through all of this even though it takes time away from being with her and James (I write this now as she sleeps next to me). She enjoys seeing me challenge myself. I have fallen in love with her even more because of it; she is a beautiful woman growing more beautiful daily. In that regard, author Daniel Pink is right. It’s really amazing what you can accomplish when you are properly motivated. It’s not always about the money or the notoriety, but rather the self-fulfillment. It’s why we have Wikipedia. So Liz, thank you so much. What you saw in me that night in Sea Isle, I still don’t know. But I’m glad you saw it.

Also when writing, I often think about James. I think about how when I am gone, when he is no longer a chunker, a pooper, or a stinker (but still a laugher and a cuddler), he will have this. These posts are time capsules, snapshots in time. They are the little pen marks on the door jamb showing how much I have grown. And maybe, one day, hope upon high hope, his pen marks will be greater.

I recently changed jobs and my former boss gave me a plaque as a going away gift. It’s made of wood, hand-painted blue, with a picture of a dog standing on its hind-legs trying to sniff a butterfly perched on a window sill. The caption reads “Always stay curious.” If there is any caption to my life, it is that. So I will continue to write. The passion that Liz and James, my parents, my sister, my friends have grown in me, I will try to impart on this screen. Because that butterfly is always there. And I need to know where it’s going next.

I hope there are some who will follow me. One of these days, it will lead to the Super Bowl. I’m sure of it.

The Preakness 2013 Interactive Simulation

I’m straying from my typical Philadelphia Eagles content here, but in addition to my love for the Birds, my dad has instilled in me a love for the track. Combine that love with an affinity for math and a glutton for punishment, and what you get is a feeble attempt at projecting horse races (unsuccessfully… I share my failures). First up, the Preakness. Click on the image below in order to interact with the simulation. The simulated results will change as you adjust the factors based on what you feel is important. Adjustable factors include Pace, Track Conditions, and Jockey Impact. Please note that this is very much a work in progress and I’m open to feedback. I may add more factors and additional functionality later, so check back for updates before the big race, as well as additional simulations for more races throughout the year.

If this simulation is any indication, things look quite interesting for Orb’s Triple Crown quest. Enjoy.

May 18, 2013 UPDATE: Added Trainer Impact as a filter/factor in the graphic.

June 6, 2013 UPDATE: Click here for the 2013 Belmont Stakes simulation.

Preakness 2013 Simulation

Chip Kelly Wants Big Guys, But Do They Matter?

Originally published April 12, 2013 on Section215.com.

Philadelphia Eagles head coach Chip Kelly has said many times that he wants tall and long football players because, according to his experience, height provides a substantial advantage on the football field.  In other words, “big guys beat up little guys”.  However, as he transitions to the NFL and modifies his philosophies accordingly, will height afford him the same advantages in Philadelphia as they did in Oregon?  Read more…

Simulating the 2013 NFL Draft with More Chaos

Originally published April 3, 2013 on HighPhive.com

Prior to free agency, I used a chaos theory-influenced draft simulation to demonstrate how accurately predicting the NFL Draft is an impossible task.  Admittedly, it was not chaotic enough, nor was it robust.  Thanks so much to reader feedback from multiple forums, I modified the simulation by… well… by adding more chaos.  The result, I think, is a better illustration of the chaos inherent within the NFL Draft.  Read more…

Projecting the Eagles’ Record for 2013 Using Pythagorean Expectation

Originally published March 26, 2013 on Section215.com

Pythagoras wasn’t quite around for tailgating, fight songs, cheerleaders, and Super Bowls, but Bill James was.  James is a historian, mathematician, and statistician whose work has greatly influenced the evolution of baseball (see sabermetrics), and has had an impact on other sports as well.  One of James’ most widely cited formulas is the Pythagorean Expectation, a formula that has been adopted by Pro-Football-Reference.com and FootballOutsiders.com, among others, and can be used to project the 2013 Philadelphia Eagles’ record with a relatively fair degree of accuracy.  Read more…

During Free Agency, the Eagles are Establishing Expectation

Originally published March 18, 2013 on HighPhive.com.

There was another year in which the Philadelphia Eagles retooled their roster for younger players who fit the ideology of its new, innovative head coach eager to install his defense. It was 1986, when former Chicago Bear’s defensive coordinator Buddy Ryan arrived with a trumpet in hand, proclaiming to the world that Philly had a winner in town.  Read more…