Longtime POTH readers know I am an aspiring defensive back. Hence, I will typically be biased against many DPI calls, but I recognize that DPI does genuinely occur. My concern about DPI enforcement in the NFL is that it is a spot foul. The offensive team is awarded a first down at the yard line where the DPI was committed. It thus assumes that the receiver would have caught the pass were it not for the DPI. A spot foul 10 or 15 yards down field seems reasonable to me, but 30 or 40 yards seems like far too much field position to simply gift the offense on what may or may not have been a catch if there were no PI. In other words, it is unfair to award the offense, say, 40 yards because of DPI given that one of several events could have led to an incompletion if there were not DPI. My thesis is that NFL DPI penalty yardage becomes increasingly unjustifiable as the spot of the foul gets farther from the line of scrimmage. But, I don’t know that and that’s why I’m exploring the matter.
any act by a player more than one yard beyond the line of scrimmage that significantly hinders an eligible player’s opportunity to catch the ball |
I needed play-by-play data. It had to include depth of target, the distance from the line of scrimmage to the yard line where the pass is caught or comes closest to the targeted receiver. I found nothing in the open-source arena but ArmchairAnalysis does provide a sample of their thorough NFL charting data, which I used. Specifically, it is a sample of about 4013 plays from two weeks in the 2019 NFL season. Of those, about 2430 are passing plays. I removed 82 throwaways, 157 sacks, and 11 spiked balls, because these events preclude a pass to a receiver. This left 2180 passing plays eligible for analysis.
First, I compared the completion % of the 2273 passing plays that were not sacks, which is 65%, to the NFL average % for all other weeks in 2019 (through week 12), which is 63.8%. This way we can assess if this sample of passes is somehow dissimilar from passes in the remainder of the season. It was not, χ² = 1.49, p = 0.22, 95% CI [0.63, 0.67].
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| Figure 1. Top panel shows distribution of completions (magenta) and incompletions (brown) by depth of target. Bottom panel shows completion % by depth of target (dotted black) and estimated probability of completion by depth of target (green). |
Figure 1 shows the raw completion percentage by depth of target (dotted black) and the estimated probability of a completion when accounting for random variance due to defense and targeted receiver (green).1 The farther a targeted receiver is from the line of scrimmage, the less likely the pass is to be completed. Indeed, this provides some support for my thesis that awarding a first down at the spot of the DPI is increasingly unjustifiable when the depth of target is farther and farther from the line of scrimmage. This is because passes targeted farther down the field are simply less likely to be completed.
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| Figure 2. Expected yards per pass attempt by depth of target (dotted black) and the expected yards when controlling for random variance due to defense and offense units (orange). |
Another way to frame the issue is in terms of yards per attempt (Y/A). That is, how many receiving yards are expected on a pass to a given depth of target. Y/A is a widely used measure of passing efficiency. Figure 2 shows the Y/A by depth of target (dotted black) and the expected Y/A when accounting for random variance due to defense and offense (orange). This provides additional support for my thesis that awarding a spot foul for DPI is increasingly unwarranted when the DPI is farther from the line of scrimmage. For example, a target of 32 yards down field is expected to gain only 15.9 yards. This might seem odd because 15.9 is less than 32, but Y/A accounts for the probability of the pass being completed. Again, passes target farther down field are less likely to be caught thus a spot foul is less justifiable.
Summarily, this exploratory post yields evidence suggesting that the penalty yards awarded for DPI in the NFL are unwarranted. Although some random variance was accounted for in the models, the major shortcoming is that other factors that affect completion percentage and receiving yards were not accounted for in the analysis. This includes factors such as QB pressure, pass coverage, field position, score differential, and others. Nevertheless, the results demonstrate that, by being a spot foul, the penalty yards awarded to the offense following a DPI (with an uncaught pass) in the NFL are incommensurate with the yardage that would be expected given the depth of target. We here at POTH have no delusions that the enforcement of penalty yardage for DPI will be subject to change. Likewise, we are not anarchists; we respect the game and know that parameters are needed to standardize competition. However, we do feel it necessary to present evidence that directly contradicts any notion of rules designed to ensure a fair game that is decided on the field, by the players.
1 Computed using GLMM specifying a binomial distribution. Depth of Target is fixed effect, with defense and targeted receiver as random effects. QB and offense were considered as random effects but were essentially null and excluded from the model. The model explained about 12% of the variance in completion %. Depth of Target was significant, reducing the log odds of completion by -0.059 for each one yard from the line of scrimmage. |
2 Computed using LMM. Simple linear regression had R2 = 0.108 and a smooth regression line had R2 = 0.11, so I used a linear model for simplicity. Depth of Target is fixed effect, with defense and offense as random effects, each with a random intercept for depth of target. The model explained about 18.4% of the variance in A/Y. Depth of target was significant, increasing the A/Y by about 1.03 yards for every three yards of depth of target. |
