Does Interception Rate Persist?

The Eagles had a very successful 2013 season. Now we need to evaluate it. After a success, particularly one as resounding as we experienced this year, the most important question to ask is:

Were they lucky or good?

Obviously, if they were very lucky, then success next year is less likely. Heading into this season, I was one of the few Eagles writers/bloggers to predict anything resembling what actually happened (I had them at 9-7, but I was just 20 points off on the point differential). Part of the reason I was so bullish was that the Eagles had bad luck last year, especially as it relates to turnovers.

Now, we have to take the same view of things. Today, I’m going to focus on one particularly important statistic from this season:

Nick Foles has an interception rate of 0.6% this season. (The single season record is 0.4%)

Aside from the obvious (interceptions are bad), this carries additional weight because if factors into whether or not Foles can be a "franchise" QB. I personally do not think he’s a great QB (or likely to become one). However, I do think he’s "good enough". The other side of the argument is that he lacks any truly elite skills. Most apparently, his arm strength isn’t great and he’s slow. His accuracy seems very good, but it’s much harder to judge that type of attribute than something more measurable like strength. As a result, while watching him play, it’s much easier to focus on what he CAN’T do or isn’t doing than on what he is doing. In light of that, allow me to posit the following:

It’s possible that Nick Foles’ "elite" quality is the ability to avoid interceptions without abandoning downfield throws. It’s possible he just has an excellent internal sense for the risk/reward of each throw. Or, if you think back to my blitz post (windows v. time), he might just have a very good sense of when a window is large enough for his skill level.

If that’s true, than I don’t see any reason why he can’t be an "elite" QB. Of course, we don’t know if that’s true and, on balance, it seems unlikely.

Today, let’s take a very preliminary step towards testing it. As the title suggests, I think the best way to proceed is to see if Interception Rate persists over time. In other words, how much does a QBs interception rate one season tell us about his rate the following season. If it does persist, then avoiding interceptions is likely a skill and we can feel really good about Nick Foles. If it does NOT, then we’re in trouble, because Foles’ amazing statistics this year were built primarily upon not throwing interceptions.

The Sample

There are a number of issues with trying to test interception rate persistence, so before we even get close to a result, we need to remember everything here is just informative rather than solid proof (I’ll explain the problems below).

To get a preliminary look, I selected 13 active QBs. The only prerequisite was that they had to have started for at least a few years. Of course, this introduces our first source of bias, survivorship. However, we’re looking at persistence, so that means we need careers that allow us to track over time. One-two year starters don’t help much (or at all). Anyway, here are the QBs I included:

Peyton Manning, Eli Manning, Drew Brees, Matt Stafford, Philip Rivers, Tom Brady, Carson Palmer, Ben Roethlisberger, Tony Romo, Matt Schaub, Michael Vick, Matt Hasselbeck.

Then, I removed any season in which the player did not have at least 100 pass attempts. For example, Tom Brady had an interception rate of 0 in 2008….because he only threw 11 passes before getting injured.

From there, I matched each player’s season interception rate with their rate the following season, ending up with 111 matched pairs.

The Result

Hmm….not what Eagles fans wanted to see. The correlation value is 0.12, so real but relatively weak. In other words, a good interception rate one season was not very likely to result in a good interception rate the following season. OR, interception rate is composed of some skill plus a fair amount of luck (that sounds about right).

I mentioned one issue with this analysis above (sample bias), but I want to mention another big one here. We haven’t accounted for defensive strength. It’s possible (likely in fact), that good defenses intercept passes at a higher rate than bad defenses. Some of the variation in QB Interception rate is therefore explained by differences in the year-to-year schedule (which are largely random).

As I said, informative not dispositive.

A few more things

After collecting the data I looked at it from a few other angles, which led to a few interesting takeaways.

- Of the 13 QBs I looked at, the largest single season deviation from their overall average interception rate (NOT career because it’s not weighted) was 2.55%. That was from Matt Stafford’s rookie year, when his interception rate was 5.3%. The second highest deviation was 2.23%. That was from Peyton Manning’s rookie year, when his interception rate was 4.9%

In fact, 4 of the 13 QBs recorded their highest seasonal interception rate in their rookie years. Moreover, another 4 of them had rookie interception rates than ranked as their second worst season. So together, 8 of the 13 QBs had either their worst or second worst interception rate their rookie seasons.

That doesn’t really TELL us anything, but it certainly suggests that QBs may improve their ability to avoid interceptions over time (which matches the "conventional wisdom"). That, of course, would be great for Nick Foles, whose rookie rate was just 1.9%.

- In light of the last point, I thought it would be interesting to take a look at the progression of each QB’s seasonal interception rates. Maybe from one year to the next there is a lot of variation, but over time QBs generally get better (or plateau around their "true" skill). Here are some individual charts, pay close attention to the X-Axis label changes if you’re comparing:

Wow…now that looks interesting. Every one of them has seen a clear downtrend in interception rate from season to season. Of course…it wouldn’t be a QB breakdown without Eli Manning:

He really does ruin everything….(and he throws a LOT of interceptions; compare his X-Axis to the others).

- The key to remember, though, is that Nick Foles registered an interception rate of 1.9% in his rookie year and just 0.6% this year. His career rate is now 1.2%.

That means, even if he is due for some regression, he’s got a lot of room to work with. He could triple his career rate next season and still be at just 3.8%. That’s high, but every QB in the sample except Rivers, Brady, and Rodgers, have hit that level at least once in their careers.

Additionally, if Nick Foles can IMPROVE his rate over time, as the QBs I showed above did, then he really will have an identifiable "elite" skill. That’s probably unrealistic (you just can’t get much better), but remember that an improvement in skill would counteract the regular variance he’d expect to see.

- A lot more data to look at regarding Interception Rate, but for now I’d say the takeaway is this:

Nick Foles is very likely to throw interceptions at a higher rate next season than he did this year. However, I wouldn’t bank on a massive shift, and given where his career rate is, I STILL expect him to finish the year with a very good interception rate (< 2.5%).

That’s good news for Eagles fans.

This post originally appeared at EaglesRewind.com. @EaglesRewind