Team performance in the red zone is obviously very critical. Teams can make up for a lot of deficiencies if they can move the ball inside the opposition's 20 yard line. But is the red zone real? Certainly, it's tougher to move the ball efficiently inside the 20 because the field is compressed and the defense has less territory to protect. As a consequence, outside the red zone the 2007 league average pass completion rate was 63%, but inside the red zone it was only 56%. So in that sense, the red zone is real.
In this article, I'll compare 2007 quarterback performances inside and outside the red zone. After testing the differences statistically, we'll see if some QBs really have a knack for success inside the 20, or if we're just witnessing the randomness of a small subsample of passes.
Football experts saw a correlation in red zone performance and scoring, and quickly focused a lot of attention on how well a team does inside the 20. It does make sense, to a degree. Teams that are otherwise efficient and can move the ball, but don't put it in the end zone won't score and won't win. A quarterback whose stats are particularly good in the red zone is considered a clutch performer and is believed to have some special ability to perform when it counts most. Likewise, a QB who under-performs in the red zone is seen as lacking what it takes to succeed in the NFL.
"My theory is that the very same abilities that lead to success in the other 80 yards of the football field also lead to success inside the 20."
But what if red zone performance was really just a random subset of overall performance? If we arbitrarily divided the field into any other 20-yard segment and analyzed performance, would we find that some QBs have a special talent between the 40s? After all, pass attempts inside the red zone comprise only 13% of all passes.
My theory is that the very same abilities that lead to success in the other 80 yards of the football field also lead to success inside the 20. A QB who is accurate, aware, can read defenses, and has a strong arm will likely do well in any part of the field. Someone would have to convince me that there is some special talent that becomes more important in the red zone.
Some might say that abilities or flaws are magnified in the red zone because of the compressed field--the density of pass defenders is much higher. If that's true, we should see quarterbacks with good stats do especially well, and below-average quarterbacks should do especially poorly in the red zone.
But that's not what we find. The #1 QB in completion percentage in the red zone last year was Sage Rosenfels. Outside the red zone however, he ranked 20th. Trent Edwards was 29th outside the red zone, but 3rd inside. Looking at the rankings, they seemed very random.
Although completion percentage is generally not the best measure of QB performance, it might be particularly relevant in the red zone for several reasons. Yardage measures are problematic because passes thrown into the end zone are truncated at the goal line. Efficiency measures may not make sense because a 2 yd pass completion from the 2 on 3rd and goal is more meaningful than a 4 yd completion from the 18 on 3rd and 8. Over the course of an entire season and across a full 100-yard field, yards per attempt might tell us a lot more than within a select set of passes near the goal line. In general, a completion in the red zone is always better than an incomplete pass or an interception. Close to the goal line, there isn't often a better or deeper option--any completion is good.
To test whether red zone completion percentage is a special talent or simply a random subsample of overall completion percentage, I used a statistical test known as a t-test. Those familiar with statistics and regression know this is the test that indicates if a variable is significant or not. But it can be used on its own without a regression to test if observed differences are due to a systematic effect or just due to random variation or sample error.
The t-test produces a probability that the observed differences are really just due to chance. Generally, a p-value below 0.05 means a variable is significant--there is a 95% chance there really is a systematic connection between variables. For example, if Matt Cassel completed 4 out of 5 passes in one series of relief for Tom Brady, does that make him more accurate than Brady's 398 for 578? The t-test says no--the sample size is too low and the difference is not big enough to conclude Cassel is more accurate than Brady.
So I performed a t-test on each quarterback's completion percentage inside and outside the red zone. But there was a wrinkle. We already know it's tougher on every quarterback in the red zone because of field compression. We're really interested in whether some QBs significantly over- or under-perform their overall completion percentage. To compare apples to apples, I used a special version of the test that accounts for known expected differences in the means of each group. The NFL average completion percentage is 7.2 points lower for the red zone than outside the red zone, so I used 7.2% as the difference in means.
The table below lists the leading QBs in completion percentage. Their completion percentage in the non-red zone (NRZ) and inside the red zone (RZ) is listed. Next is listed a "value over average" (VOA) number that indicates how a QB over- or under-performed his overall percentage when inside the red zone, accounting for the general increase in difficulty inside the 20. A high positive number means a QB did especially well in the red zone compared to outside it. The final column is the result of the t-test. A number below 0.05 is generally considered significant.
Click on the table headers to sort.
|QB||Team||NRZ Pct||RZ Pct||RZ VOA||T-test|
Note that there are in fact three QBs with statistically significant differences in completion percentage inside the red zone. Brees, Edwards, and Rosenfels all had significantly better than expected performance in the red zone. No QBs were significantly worse than expected. So this is proof that those three QBs have a special ability to perform in the compressed field inside the 20, right?
"It appears that despite all analysis to the contrary, there is nothing special about any particular quarterback's ability inside the red zone as compared to outside the 20."
From this, it appears that despite all analysis to the contrary, there is nothing special about any particular quarterback's ability inside the red zone as compared to outside the 20. It's simply a random subsample of overall performance. If a QB is a good passer, he'll probably be good inside the 20.
If this conclusion is correct, it should adjust our view of some of the QBs in the table above. (Sort by the red zone value over average (RZ VOA) column to see the most over- and under-performing QBs.) Take Trent Edwards for example. His over-performance inside the red zone probably makes him appear to be more productive a passer than he truly is. He's not likely to over-perform inside the 20 in 2008 like he did in 2007. Chances are Buffalo fans will be disappointed next year.
The opposite could be said of QBs such as Kellen Clemens. He under-performed in the red zone in 2007. Although his overall numbers aren't terribly great, chances are he will not under-perform inside the 20 as severely as he did last season. It's not a guarantee Clemens will improve, but he would likely trend toward his overall performance level and not repeat his dismal 39% completion rate in the red zone.
This result also means that prediction models and handicappers that overweight red zone performance are reducing their predictive power. They are chasing the past randomness of a small subset of performance. The same is probably also true of the myriad of stats "splits" such as "under the lights" or "road division games " or any other silly and arbitrary classification.