January 10, 2012

Completion Percentage Vs. Yards Per Attempt

     I've always said that completion percentage is the most overrated statistic in football, if not all of sports.  It's the first number cited whenever somebody talks about how good or bad a quarterback is.  Yet the actual act of completing a pass is not that tough.  I could complete 100% of passes if you asked me to, but they would all be to someone two feet in front of me and wouldn't be effective at all.  Completion percentage doesn't really tell you how good a QB is.  It's more of a description of the QB's style and the type of offense he runs.  That's why I always look at yards per attempt.  You can manipulate completion percentage, but you can't really gain yards without being productive.

     Case in point:  Tim Tebow's performance against the Steelers.  His Total QBR, an overall rating used by ESPN that I still hate, was 97.3, the highest in a playoff game since at least 2008.  This was despite Tebow only completing 48% of his passes.  He did, however, pass for 15 yards per attempt, more than double the league average.

     Of course, I was wrong about the BCS being any good at picking a champion, so maybe this topic needs to be looked into as well, using my brand of technically unsound mathematics.

     Using the drive simulator from the Pass vs. Run post, I created two scenarios.  The first kept everything at the NFL's average through Week 10, except for completion percentage.  The percentage was switched between 7 different values, with the other numbers being adjusted so that yards per attempt and interception percentage would stay the same.

     There is a clear advantage to completing passes and getting yardage consistently rather than in big chunks.  But the difference between an awful 20% and a wonderful 80% is only 0.66 points per drive, a fairly small number.

     The second scenario fixed all numbers except for yards per attempt, which was also switched between 7 different values.

     The gap appears to be much wider here, with 1.48 points per drive separating 4 yards per attempt and 10 yards per attempt.

     In the NFL's formula for passer rating, 1 yard per attempt is worth 5 percent in completions.  After adjusting the point totals (by removing one point per interception), one yard per attempt is found to be worth 0.2533 points per drive.  To change the output by that same amount, you would have to adjust the completion percentage by 24.23.  This means that the passer rating formula overvalues completion percentage by a factor of 5.

     Don't misunderstand me; completing passes is important.  But the reason it's important is that it makes achieving high numbers in other categories a lot easier.  All I want is for people to realize that completion percentage on its own is pretty much useless when looking at the skill level of a quarterback.  It's the other numbers like yards and interceptions that affect the outcome of the game.

1 comment:

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