A Statistical Look at How Turnovers Impacted the NFL Season
“Turnovers are like ex-wives. The more you have, the more they cost you.” – Dave Widell, former Dallas Cowboys lineman
It doesn’t take witty insight from a former NFL player to realize how big an impact turnovers can have in a football game. Every time an announcer talks about “Keys to the Game,” winning the turnover battle is one of them. And as Cowboys fans know all too well, an ill-timed interception can ruin not only your chances of winning that game, but it can ruin your entire season, too.
But hold on a minute. A few weeks ago, Andrew Luck and the Colts proved that you could still win a game despite having 3 more turnovers than the opposing team. In fact, teams that lost the turnover battle are 4-4 so far in the NFL playoffs this year. So is it possible that we overvalue the importance of turnovers, or are the 8 playoff games I’m looking at just a small sample?
How Much of an Impact Do Turnovers Have on a Team’s Season?
Obviously in any one game, a team can lose the turnover battle and still win the game. But is that sustainable over the course of a 16-game NFL season?
For all 32 NFL teams, I recorded their turnover differential, their regular season winning percentage, and their regular season scoring differential. I want to see if having a positive turnover differential (defined as your defense creating more turnovers than your offense commits) led to a higher winning percentage and scoring differential. You can get the data here.
I used Minitab to create fitted line plots of the data (Stat > Regression > Fitted Line Plot...). Let’s start by comparing a team’s turnover differential to their winning percentage.
There is a positive correlation between turnover differential and winning percentage. We can conclude that 44% of the variation in a team’s winning percentage can be explained by their turnover differential. This shows that turnovers are not overvalued when it comes to winning percentage. Andrew Luck may have won a single game with a turnover differential of -3, but this plot shows (as did his next playoff game against New England) that you can’t continue to turn the ball over and win football games.
But we shouldn’t rest our laurels on winning percentage. Scoring differential is a better indicator of how good a team really is, so let’s see if the same trend holds when we use that as the response instead of winning percentage.
This graph looks almost completely the same. It doesn’t matter if we look at winning percentage or scoring differential, turnovers play a major part in both. And one thing I’d like to point out is the Denver Broncos. Despite having a turnover differential of 0 for the entire season, they still outscored their opponents by over 200 points. Considering Peyton Manning threw only 10 interceptions (only 4 teams had fewer all season), it’s scary to think of what that team could have done if their defense actually created some more turnovers!
Are Turnovers Skill or Luck?
We just saw that teams with a better turnover differential have a better record and scoring differential. But are those teams with a high differential skilled at generating/avoiding turnovers, or are they just lucky? To answer this question, let’s start with a defense’s fumble recovery. I took the number fumbles recovered by a defense through the 1st half of the NFL season, and compared it to the number of fumbles they recovered the 2nd half of the season. If causing and recovering fumbles is truly a skill, then defenses with high recoveries in the 1st half of the season would continue to do so the rest of the season.
This clearly shows that there is no skill in causing and recovering fumbles in the NFL. The number of fumbles a defense recovers in the 1st half of the season explains 1.6% of the variation in the number of fumbles they recover in the 2nd half of the season. It’s completely random. But even though teams have no control over their fumble recoveries, we saw previously that it plays a huge part in their record.
Take for instance, the Pittsburgh Steelers (top left corner).
The Steelers started the season going 2-6, with their defense recovering only 1 fumble (worst in the NFL). But in the 2nd half of their season, their luck changed, and they recovered 9 fumbles (best in the NFL). Not surprisingly, their record dramatically improved in those final 8 games to 6-2. Now, the fumble recoveries don’t fully explain their turnaround, but it definitely played a big role. And it’s a much more tangible explanation than banning games in the locker room.
Ok, so it’s really no surprise that fumble recoveries are luck, but what about defensive interceptions? Surely a team with a really good secondary should have a higher interception rate than those with a less talented secondary. Let’s see what the statistics say!
Just like fumbles, defensive interceptions are completely random! Look at Tampa Bay at the top of the plot. After the 1st half of the season they had only 6 interceptions and were 0-8. Would anybody have predicted that in the 2nd half of the season they would more than double their interceptions and have as many as the Seattle Seahawks (considered one of the best defenses in the NFL)? Of course not! Oh, and after going 0-8 to start the season, Tampa Bay finished 4-4. Their increase in interceptions was a major factor, as 11 of their 15 interceptions in the 2nd half of the season came in their 4 wins. It’s easy to look back in hindsight and say those interceptions explain why they won. But this plot shows that defensive interceptions are so random that it is impossible to predict in which games they will occur.
Another thing I’d like to point out is the team on the lower left-hand corner of all 3 plots we’ve looked at. It’s the Houston Texans. They weren’t able to force many fumbles or interceptions in either half of the season, and not surprisingly had the worst record in the NFL! No matter how bad your defense is (not to mention one with J. J. Watt on it), it’s extremely unlucky to get so few turnovers. Heck, even the Jacksonville Jaguars defense had 8 interceptions in the 2nd half of this season! Next year the Texans will have a new head coach in Bill O’Brien, and will likely draft a new quarterback with their #1 overall draft pick. If they improve on their 2-14 record (and they will) much of the credit will be given to O’Brien and the new quarterback. But considering how unlucky Houston was with defensive turnovers, their record is bound to improve on luck alone!
Overall, these plots lead us to believe that great defenses hold opponents to fewer yards and fewer points. But causing turnovers? That’s just lucky icing on the cake!
I’m not going to bore you with offensive fumbles (spoiler alert: they’re random too!). Instead I’m going to look at something that should have some correlation from the 1st half of the season to the 2nd...offensive interceptions. Surely if Peyton Manning or Tom Brady throw a low number of INTs the first half of the season, they will continue to do so in the second half. Likewise teams with bad quarterbacks should keep throwing INTs the all season long! Let’s see if the statistics agree.
There is a weak positive correlation between interceptions in the 1st and 2nd half of the season. So of all the turnovers that we looked at, this is the only one that isn’t completely random. The small R-squared value shows that interceptions can still be erratic, but there is at least some skill in not throwing interceptions.
Want to see why the New York Giants were so bad this season? Just look at the top right corner. But unlike the Steelers, Buccaneers, and Texans, who just got unlucky, the Giants have somebody they can pin the blame on: Eli Manning. While his older brother had arguably his best season ever, Eli had by far his worst.
Fun Football Narratives!
Now lets look across the plots to see how luck told the fortunes of two different teams, the Detroit Lions and Cincinnati Bengals.
In the first half of the season, Detroit Lions quarterback Matthew Stafford threw only 6 interceptions, which is just as many as Tom Brady and Peyton Manning. The Lions were 5-3 and in great position to make the playoffs. But Stafford self-destructed in the 2nd half of the season, throwing 13 picks. In addition, the Lions defense had only 5 interceptions (6th worst in the league) and only 4 fumbles (bottom half of the league) in the 2nd half of the year. When you combine the bad play of Stafford with the unlucky Lions defense, it’s no surprise that the Lions went 2-6 in their last 8 games and missed the playoffs.
Cincinnati Bengals quarterback Andy Dalton had almost the exact same season as Stafford, throwing 7 picks in 1st half of the season and leading the Bengals to a 6-2 record. And just like Stafford, Dalton threw a disastrous 13 interceptions in the 2nd half of the season. But his poor play was masked by the Bengals defense in the 2nd half, which had 13 interceptions (3rd best in the league) and 6 fumble recoveries (8th best in the league). The Bengals went 5-3 in their final 8 games, won the AFC North, and earned the 3 seed in the playoffs.
So the Bengals and Lions both had poor quarterback play in the 2nd half of the season. But one team was able to get defensive turnovers while the other wasn’t. And of course in the playoffs, the Bengals luck ran out. Dalton continued to play poorly, throwing two interceptions and losing a fumble, while the Bengals defense didn’t force a single turnover. And just like that, Cincinnati lost a home playoff game by 17 points.
Maybe It Really Is Better to Be Lucky than Good
Although they don’t explain everything, turnovers can go a long way in determining whether a team wins or loses. But for the most part, the frequency at which they occur is very random. And keep in mind that we looked at the entire NFL season. Imagine if you cut that sample down to one game, such as the AFC or NFC Championship! This Sunday four relatively even teams will play for a chance to go to the Super Bowl. Don’t be surprised if the winning teams are victorious because they got a lucky bounce on a turnover. But good luck trying to figure out which team will get that bounce!
This data analysis showed us that turnovers are great at explaining why a team won or lost. But they are so random, it’s almost impossible to use them to predict who will win a game!