The Persistence of Sack Rate

Before we get to today’s topic, I need to clear something up about the last post.  It seems as though many readers came away with the wrong message. Nobody here (EaglesRewind.com) falls into that camp (at least nobody who commented), but Philly.com pulled the post from BGN and ran it on the front page of the site….

Hilarity ensured, at least if you read the comments.

So, let me make this clear:  I am a huge Nick Foles fan and the last post was not really bad news for him.  Yes, interception rate shows very little persistence from one year to the next, and that is, by itself, bad news for Foles (because he did so well this year and last).  However, I also mentioned that, (1) Foles rate was so good that even a relatively large amount of regression would still leave him with a very strong rate, and more importantly, (2) it looks as though some QBs are able to improve upon their interception rate over time.  That second part is a vital piece to this puzzle.  It means there is definitely skill involved in avoiding interceptions (hardly a surprise), but it also means that Foles may be able to actually improve his “true” ability level, which would obviously counteract some or all of the expected regression.

So, good news, not bad.

Now, for today, I decided to look at another aspect of Foles’ game thats drawn a lot of scrutiny: his sack rate.  

Foles has a career sack rate of 7.6%, and this season he was sacked on 8.1% of his drop-backs. (Pro-football-reference.com).

The first question, obviously, is:  Is 8.1% bad?

Well, it’s not good.  This season, it left Foles ranked 27th in the league, just behind Kellen Clemens.  There were a few notable QBs who did worse though, like  Cam Newton (8.3%), Colin Kaepernick (8.6%), and Russell Wilson (9.8%).

I know what you’re saying, those are all “running” QBs, and their high sack rate should be balanced against the positives they bring in the running/scrambling game.  (If you weren’t saying that, you should have been).  While I’ll attack the positive/negative balance another time (check Football Outsiders’ advanced metrics for a very surprising look), allow me to posit another potential explanation:  they’re all relatively young.

Here is the list of QBs who finished with a worse sack rate than Nick Foles this year, along with their ages (right column).

Screen Shot 2014-01-27 at 10.07.45 AM

That’s a lot of 23 and 24.  This makes complete sense, for one of two reasons.  Either:

1) Sack avoidance is a skill that can be improved over time

OR

2) QBs who take a lot of sacks don’t remains starters for very long.

The most likely explanation is that its a bit of both (as it usually is).  If a QB takes sacks at a very high rate, it’s going to be very difficult to be a productive offensive player.  There are exceptions, though, like the QBs I named above (Foles, Wilson, Kap, etc…).  Also, Ben Reothlisberger has a career sack rate of 8.2%.  Clearly, a high sack rate is not catastrophic.

For more, look at this year’s leaders:

Screen Shot 2014-01-27 at 10.12.44 AM

Jason Campbell, Jay Cutler, Sam Bradford, Matt Schaub, Matt Cassel….There are a lot go great QBs on that list, but also a handful of mediocre-bad ones as well.  Once again, sack rate isn’t everything.

Now, to the title of this post, persistence.

In my last post, we found that interception rate persists at a relatively weak level (at least within our admittedly limited sample).  That means one year’s rate has little informational value regarding the following season.

Let’s do the same thing with Sack Rate.  Simply put, does getting sacked at a high rate one season (like Foles this year) mean you’re relatively more likely to be sacked at a high rate the following season.

The answer, overwhelmingly, is YES.

Here is the chart:

Screen Shot 2014-01-27 at 10.18.09 AM

Again, the sample only consists of 100+ attempt seasons from: Eli Manning, Peyton Manning, Matt Schaub, Mike Vick, Matt Hasselbeck, Drew Brees, Tom Brady, Philip Rivers, Matt Stafford, Carsoln Palmer, Tony Romo, and Aaron Rodgers.  All told, that’s 112 qualifying seasons.  The same caveats I discussed last time apply here as well.

From the above data, the correlation value is 0.57, which is VERY strong compared to most statistics in football.

Clearly, a player’s style and skill have a large effect on their sack rate.  More importantly, Nick Foles is likely to be sacked at a high rate next year.  By itself, that’s bad news, but not as terrible as it might seem.

First off, the youth factor.  Here is a chart showing the average sack rate of the entire sample by qualifying season.  Note that these are NOT weighted numbers, so Hasselbeck’s 10.6% rookie rate counts the same as Aaron Rodgers’ 6%, regardless of the number of attempts.

Screen Shot 2014-01-27 at 12.23.39 PM

 

There is a clear and significant drop-off after the first season.  Of course, this year was Nick Foles’ SECOND season, but he did start just 6 games his rookie season, so I don’t think it’s unfair to look at this year as something like a continuation of his first season (though we can’t ignore the fact that a full offseason SHOULD play a big role).  There are definitely a lot of other factors in play here, for example maybe teams tend to improve the offensive line in the first offseason after anointing a new QB.  So nothing here is definite.

Still, from the sample I looked at, it’s certainly possible, and perhaps likely, that Nick Foles will see a decline in his sack rate next year.  However, the high correlation value suggests that we shouldn’t expect a significant difference.

I’m going to leave it there for now because I already feel like I’m rambling a bit, but let me give you one last chart and correlation value to think about.

How do interceptions and sacks relate?  The “narrative” frequently says that it’s “better” to take sacks than to throw interceptions.  Alone, that’s almost always the case.  But, it seems to suggest that one can and does take sacks INSTEAD of throwing interceptions.  For example, everyone that says Nick Foles doesn’t throw interceptions BECAUSE he’d rather take a sack is making a logical jump and assuming that the two outcomes are related.  I need to do A LOT more research on this, but preliminarily, here’s a chart showing the correlation between interception rate and sack rate, within our sample:

Screen Shot 2014-01-27 at 12.33.41 PM

 

A jumbled mess.  The correlation value is just 0.123 (roughly the same strength as the persistence of Int rate).  Remember, if the “Sacks INSTEAD of INTs” narrative holds, we should see a negative correlation (so more sacks equals fewer interceptions).  That’s not what we have.  As explained, I need more data and a deeper dig before coming up with any conclusion I can feel comfortable about.  In all likelihood, we need to groups QBs by “type” and evaluate them within discrete groups according to their attributes.

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2 thoughts on “The Persistence of Sack Rate

  1. Great work as usual Brent. I was thinking to myself while reading the article that I would love to see a sack v. interception chart, and there it was at the end waiting for me. My hypothesis is that quarterbacks fall into four quadrants: 1) high, high; high, low; low, high; and low, low. over the course of their careers, quarterback probably tend to gravitate towards either high, high — leading to them getting replaced — or low, low, thus improving with time. By itself the breakdown isn’t revolutionary, but if we can confirm the theory we would be able to better understand the relationships between cumulative career sacks and interception rates. I’m not sure how robust your data is here and am assuming that strength of OL/WR/Def. will distort the data, but with a large enough sample size we could probably get somewhere. Let me know what you think!

  2. Thanks for the tip about philly.com – the comments there were hilarious. I particularly liked the ones that quoted basic stats like YPA and then claimed that it ‘disproved’ your article.

    Great work as always, Brent. It’s places like this that make me forget how dumb most of the internet is.

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