Everyone knows that it's important to protect the quarterback. Teams spend millions of dollars on the best offensive linemen in hopes that they will give their quarterback enough time to tear up the opposing secondary.
Since passing is so important in the NFL today, how can we measure the effectiveness of protecting the quarterback?
Sacks and Win Percentage
As a simplistic measure, let's use sacks as a measure of protecting the quarterback. Below is a plot of win percentage for every quarterback in the league that's played at least 4 games versus their average sacks per game:
There seems to be a clear trend that fewer sacks per game improves a team's chances of winning. The R-squared value for that trend is 0.29, which isn't large, but is significant as far as football statistics go.
Sacks and QB-Rating
On the other hand, there is no correlation between sacks per game and quarterback rating:
On the extremes here are Kerry Collins with only 1 sack in 7 games with a 74.2 QB rating and J.T. O'Sullivan with a bone-crushing 29 sacks in 7 games and a 75.5 QB rating.
Using sacks per game may not be the best measure of how well an offensive line protects the quarterback. I used it here because it's the easiest to measure. A statistic that incorporates sacks, hurries, and knock-downs would be a better indicator of how well a quarterback is protected.
Quarterbacks Perform Independent of O-Line?
If J.T. O'Sullivan were traded to Tennesee and played behind their offensive line, would he be transformed into a superstar? I don't think so. My guess is that his QB rating would have a slight but negligible increase.
This isn't a conclusive study, but I think this indicates that a quarterback's performance is almost entirely independent of their offensive line. There are a lot of uncontrolled variables here such as the quality of the rest of the offense and the strength of schedule, but I'm not too surprised by this result.
O-Line and Running Backs
What about Frank Gore? Would his yards per carry increase if he were traded to Tennesee? I bet it would. Running backs seem to be more interchangeable than quarterbacks and depend more on their offensive line.
Sacks allowed is somehow related to win probability, but perhaps not through quarterback performance. I'll look into the effect of offensive line on the running game in a later post. There is probably a reason that teams invest heavily in offensive lines, and there should be a way to measure how effective that strategy is.
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