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Random thoughts on completion percentage

Posted by Jason Lisk on March 11, 2010

This post will be exactly what it says--random thoughts on completion percentage as a measure of quarterback performance, as a statistic used in passer rating, etc.

1. Contrary to what you might think, I do not hate completions. Incompletions are a bad thing. A quarterback who completes 50% of his passes for 7.0 yards per attempt would be more productive if a few of those incompletions were instead turned into short completions. He might be a 55% passer with a 7.3 yards per attempt instead. A few more, and he might be a 60% passer with a 7.6 yards per attempt. Clearly, the latter is much better than the former.

2. The issue, such as what comes up with passer rating, though, is not whether incompletions are bad--they are. It's whether the tradeoff of incompletions for higher yards per completion or attempt is bad. It's whether Jason Campbell's 62.3% completions at 6.4 yards per attempt in 2008 is really as good as Mark Brunell's 57.7% at 6.7 yards per attempt three years earlier for the same franchise. Both of those seasons garnered similar passer ratings.

3. Completion percentage is often cited as a measure of accuracy. Clearly, the more accurate you are, the more passes you will complete, all else being equal. The problem, though, is that everything is not equal. It is highly dependent on the offensive system, the game situation, and the quarterback (or offensive) philosophy when it comes to attacking the defense, as well as the quarterback's willingness to throw the ball under pressure rather than take sacks. Let's put it this way. Is Chad Pennington really the most accurate passer in NFL history? To me, citing his completion percentage as evidence of being most accurate is a bit like citing the old lady who drives five miles a week as the safest driver because she has no accidents. We need more information than number of accidents or completion percentage before we can make a value judgment about safest/most accurate.

4. I don't think the relationship between completion percentage and success is linear, where we can simply say that a every additional completion is worth x points across the board. Let's consider some extreme hypotheticals. We have two teams that both average a whopping 14 yards per attempt. One team completes 100% of its passes; the other 50% (for 28 yards per completion). If I were to model those two, it seems pretty clear that the team that completes 100% would score more. They would score on virtually every possession, only failing to score in limited cases where their 3 consecutive completions net 9 or fewer yards. The 50% team would also score alot, but string together a few more droughts. I suspect my 100% completion team with 14 yards per attempt would average about 60 points a game, while the 50% completion team would average closer to 50.

At the other end of the spectrum, we have two teams that average 3 yards per attempt. If one of those teams completed 100% of their passes, they would struggle to maintain drives or even get them started, while a 25% completion team would occasionally string together first downs and get into scoring range. Neither would score much at all, but if I were forced to watch both teams for 24 hours straight as punishment for all my transgressions, I'd take the team with the yards per completion to win in a non-shootout.

Now, the question is, where is the line where it begins to switch? Where does a higher completion percentage at the same yards per attempt matter significantly in terms of points?

5. A team that completes a higher percentage (at the same yards per attempt) will have more first down passes. This is true. The comparable team with the lower completion percentage will nevertheless have a higher percentage of completions that result in first downs avoided. What do I mean by this? Consider two quarterbacks who gain 30 yards, the one that went 2 for 2 may have picked up two first downs. The other who gained it in one pass picked up one first down, and avoided the necessity of executing and gaining at least one more.

6. This one hit me when I was looking at Sonny Jurgensen's career splits as a result of this post on quarterback schedule adjustments. Jurgensen's yards per attempt, touchdown percentage and interception percentage were all worse against the Western Division, supporting the theoretical adjustment to schedule in his case. However, his completion percentage was actually higher against the West.

People talk about quarterbacks who pad their stats when the team is trailing. They'll talk about things like garbage touchdowns or late yardage to put up what appears like a big game. You never hear people talk about padding the completion percentage, though. For good quarterbacks, these late situations are precisely when they can put up high completion rates, all else equal (I'll get back to that in a second). Defenses that have a lead are willing to concede yards and first downs for time and preventing big plays late in a game. This is the situation when the accurate QB should make hay. I play fantasy football in a league that gives points for first downs. I've watched it happen as someone like Brees or Warner racks up first down after first down late when trailing. On the other hand, defenses that are trailing are incentivized to prevent completions, because they need to stop the clock and force a change in possession; a big play may be only marginally more costly than allowing a couple of first downs. In these situations, the same quarterback may find that completing passes is more difficult, but gaining larger chunks of yards is more likely to occur.

The problem with measuring this effect, though, is that good passers usually don't trail against bad pass defenses to begin with, so if they are trailing, it's usually to a pretty good defense. If we could artificially manipulate it, I suspect we would find Peyton Manning would complete a higher percentage of passes against the same Ravens defensive unit if we started two artificial games with 15 minutes left, one with the Colts trailing by 10, the other with them leading by 10.

7. Sacks taken should count against completion percentage; sacks avoided by throwing incomplete or out of bounds do. Or at least we should have a separate statistic that measures completions versus total opportunities to pass. A quarterback who throws two passes away rather than taking a sack will have a lower completion percentage, but has saved his team's chances at points. A 18 for 30 game (60%) becomes a 18 for 32 game (56.25%) with two sacks avoided-type incompletions.

8. Back to the question of where the line is for where completion percentage increase becomes advantageous for the offense. My prediction/guess is at roughly 7.5 yards per pass attempt, or roughly the point where a team that completes 65% of its passes is still going to average 11.5 yards per completion and throw for a healthy number of first downs per completion, that it becomes very profitable to have the more consistent (i.e., higher completion rate) offense compared to the less consistent one. This is probably about one standard deviation better than the average, and would mean that higher completion percentage (for teams with the same yards per attempt) is either a small factor or a negative factor for about 85% of the teams. It matters greatly for the elite passing teams, but probably not for most.

In other words, it matters at the high extreme and was a big reason why the Niners teams of the 1980's and 1990's were so good. Not so much for guys like David Carr and Charlie Frye.

9. Completion percentage, like sack percentage, is somewhat consistent for individual quarterbacks, even when they change teams. This suggests that part of what we are measuring is a repeatable skill and not just entirely system-driven or teammate related. I would tend to take the quarterback with the higher comp% over the lower comp% going forward in the future to be able to maintain the yards per attempt rate. In other words, I think it has some predictive value. This has to be tempered by system, because it is pretty clear that the QB's in a Don Coryell-based offense (Fouts, all of the Redskins QB's of the 1980's and early 1990's, and the Rams and Vermeil era Chiefs recently) are undersold by passer rating relative to adjusted net yards per attempt in terms of the value they provided, and the West Coast passers are oversold, and its because of the different philosophies as they affect completion percentage.

10. After writing paragraph #8, I pulled a list of team-pairs going back to 1970 for teams with the exact same yards per attempt in the same season, but completion percentages that differed by 5% or more. I'll probably post that separately rather than lengthen this one.

This entry was posted on Thursday, March 11th, 2010 at 8:08 am and is filed under General. You can follow any responses to this entry through the RSS 2.0 feed. Both comments and pings are currently closed.