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Under Pressure: Differentiating Pressure Rate from Sack Rate

ryantannehill

Just as a heads up, this article on its own probably doesn’t have much actionable fantasy information, but it is the beginning of my effort to quantify the effects of offensive lines. Isn’t it strange that, although we all pay lip service to “oh they added a new guard, that should help runningback x,” talking about quality of offensive line and changes therein really isn’t something that makes a significant impact on fantasy football discussion? It’s actually very strange; there’s a huge correlation between offensive efficiency and the fantasy outputs of the players in those offenses, so if NFL teams are dumping a ton of resources into their O-Lines, either we’re not paying enough attention to the effects of the hogs or the NFL is doing something wrong? Right?

Methodology

Transitions are hard, but lets talk about offensive line data. PFF keeps some interesting pressure and sack data. Basically they tell you on what percentage of snaps a quarterback is pressured and then tell you what percentage of those pressures turn into sacks.1 Pressure percentage is obviously at least largely a reflection of offensive line quality. So the question becomes: is sack percentage independent of pressure, or do high-quality offensive lines give up fewer sacks even when they do allow pressure? Using PFF pressure and sack data from 2010-2013, I found that there is absolutely no correlation between Pressure% and Sack% when pressured. This is important because if better offensive lines gave up “lower quality” pressures than worse lines we would expect to see some correlation between Pressure% allowed and Sack% allowed.What the data suggests is that, once an offensive lineman gives up a pressure, whether that pressure results in a sack is largely out of his hands.

So what does predict what percentage of pressures become sacks?

I looked at the Sack% of every QB who received over 50% of their team’s snaps in back to back years. What I found was their year N Sack% accounted for 21% the variance in that player’s sack percentage in the N+1 year. What we can posit, then, is that avoiding sacks when pressured is a quarterback skill and one that is somewhat consistent from year to year.

Why is this bad news for Ryan Tannehill?

Ryan Tannehill was the most sacked quarterback in the league last season and it wasn’t particularly close (he was sacked more times in the regular season than anyone since Jon Kitna went down 63 times in 2006.) Because of this, it’s a pretty widespread belief that the Dolphins are in desperate need of offensive line help.

Here’s the problem though; Tannehill was actually only pressured on about a third of his snaps, which was actually below the league average. The issue was that he was sacked a league high 26.1% of the time he was pressured (for comparison, the average since 2007 is about 17.5%.)  Even though it should be hard for Tannehill to stay this bad at avoiding sacks, my model suggests he will be sacked 21.3% of the time when he is pressured this year, or about 4% above the league average. As just a back of the napkin calculation, if we project Tannehill to have 600 drop backs next year (he had 661 in 2013) and to be pressured on 1/3rd of them, Tannehill’s inability to avoid sacks when pressured would be expected to result in him taking about 8 extra sacks over the course of a season, a clearly nontrivial amount.

What did we learn:

To be clear, I doubt this has any bearing on Ryan Tannehill’s fantasy value. However, I also think we have good reason to believe that avoiding sacks when pressured is a quarterback skill. So now when a team makes moves to improve its offensive line we can probably at least start to have a data-based framework for evaluating the effect of those moves.

  1. so Pressure%*Sack%=percentage of pass plays that are sacks.  (back)

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