Stat Sheet Misconceptions: Interceptions

Now that we’ve seen the flaws in completion percentage, it’s time to examine another statistic commonly used to evaluate quarterbacks. This time we’ll look at every quarterback’s worst enemy: the interception.

Perhaps the most remarkable aspect of Tom Brady’s impressive 2010 season was that he was intercepted only four times. That low pick total was a huge contribution to the Patriots’ run as one of the best teams in the NFL last year. Eli Manning, on the other end of things, had 25, with nine coming in the last five games – part of the reason the Giants weren’t playing in January.

When looking at games won, there’s no question that interceptions are a useful indicator. In 64% of last season’s games, the winning team was the one that suffered the fewest interceptions. To contrast, the winner was picked off more than the loser in just 8%. Games that saw each team throw the same number of interceptions made up the other 28%. What I will argue is that, while it's a major factor in deciding games, it isn’t as effective a quarterback-evaluating tool as is commonly believed.

 
An Interception vs. a Bad Throw

Asking what the difference is between an interception and a bad throw might seem like a silly question. A bad throw usually results in an incomplete pass, which isn’t great, but it doesn’t hurt the team that much either. At best, the offense has another down and, at worst, it’s time to punt. On the other hand, an interception robs the offense of an opportunity to score, gives the defense an immediate chance for points, and the resulting field position is usually much better than the defense would have earned by forcing a punt.
 
From the perspective of evaluating quarterback play, however, there should be no difference in judging a bad throw or an interception. Two quarterbacks could produce the same number of bad throws, but it’s largely luck that might cause one to wind up with a few more interceptions. In one situation, the defender is at the right spot at the right time and makes the catch, and in the other, he’s not. One quarterback, then, would typically come away with a much harsher review when, in reality, their performances were very similar.
 
Interceptions are really just a slice of what should be counted against a quarterback. Three percent of pass attempts in 2010 were intercepted, but we awarded quarterbacks negative ratings on more than 16%.
 
Not Always The QB’s Fault

In the same way that a dropped pass is inappropriately counted against the quarterback, there may be interceptions that occur that aren’t his fault. A look at one such interception was in Week 14 when the Lions and Packers played each other. With two minutes left in the first quarter, Aaron Rodgers threw a beautiful pass leading Greg Jennings. Placed right where it should be, Jennings bobbled the ball, and it ended up in the hands of Lions rookie safety Amari Spivey. (You can find the play here)
 
While this is an extreme example of an interception not being the quarterback's fault, there are a number of other situations that would qualify – a ball gets tipped, a receiver falls down, etc. During the 2010 regular season, there were 509 intercepted passes. On 103 of them, we gave the quarterback a zero rating because it wasn’t a poor throw. On 16 of them, including the Aaron Rodgers play, we awarded a positive grade because we felt the quarterback made a good throw.
 
In 2010, Brett Favre earned a negative rating from us for only 10 of his 19 interceptions. However, his poor play in other situations led to a relatively low overall grade. Other players that threw a significant number of non-negatively-graded interceptions were Eli Manning, Drew Brees, David Garrard, Matt Schaub and Jon Kitna. Conversely, Jay Cutler and Sam Bradford were two of the leaders in interceptions this year, and each had just one that wasn’t graded negatively.
 
Knowing the Situation

In the completion percentage article, we discussed the idea that not all passes are equal, and again it’s true when it comes to interceptions. Game situation plays a role. For example, there were three times as many interceptions thrown when in the last two minutes of a half as compared to the rest of the game. Understandable because teams are often pushing downfield with less caution in this situation, trying to get themselves into scoring range against defenses that are fully aware of their plan.
 
Again, as with completion percentage, the distance the ball is thrown affects interception numbers. In 2010, balls thrown behind the line of scrimmage were intercepted just 1.1% of the time. Between 0-10 yards, the interception rate went up to 2.0%. From 10-20 yards, it doubled again to 4.0%, and for passes over 20 yards, it jumps all the way to 8.2%.
 
Whether or not the quarterback is under pressure also contributes. Well-protected quarterbacks throw interceptions 2.56% of the time. If under pressure, however, that number goes to 3.90%. It makes sense in that quarterbacks generally make poorer decisions when facing pressure.
 
These are just a few factors that go into interception rates, factors that a glance at raw interception stats wouldn’t reveal.
 
Closing Thoughts

As I’ve stated, there is no questioning the impact an interception has in a football game. The problem is we’ve grown accustomed to taking what happens on that 3% of pass plays to be representative of all poor throws made by a quarterback. Not only must we remember that the three percent is not the total of all bad passes, but also that the entire three percent may not be the quarterback’s fault. Interceptions only tell a part of the story.