Passes Defensed on Standard Downs Compared to Passing Downs: Are Some Defenses More Aggressive than Others?

Football is largely a game of competing strategies. Aside from coaches’ philosophies, players’ executions, and those of opponents, strategizing is dictated by fluctuating situational factors within each game. One factor is garbage time which occurs when a team is winning or losing by a large margin. Here, winning offenses might play more cautiously to maintain their lead whereas losing defenses might take risks to reduce the scoring margin. Likewise, losing offenses might pass the ball more frequently attempting to score rapidly whereas winning defenses might play to prevent large passing gains. We will revisit garbage time shortly.

A second factor or set of factors influential in determining strategy are standard and passing downs. Standard downs are defined as 1st downs, 2nd & ≤ 7, and 3rd or 4th & ≤ 4. Passing downs are defined inversely; 2nd & ≥ 8 and 3rd or 4th & ≥ 5. Extreme philosophies notwithstanding, on standard downs, offenses could pass or run with equal likelihood given the yards needed to secure a first down. Alternatively, on passing downs, offenses should be more likely to pass the ball given the yards needed to secure a first down. In college football the distinction is generally reflected statistically: Prior research indicates that offenses will pass on ~40% of standard downs and on ~67% of passing downs.

I am curious if passes defensed (hereafter: PD)—pass break-ups and interceptions—occur more frequently on passing or standard downs. Defenses should be attuned to what is coming on passing downs (i.e., a pass), should adjust their personnel (e.g., more men in coverage), and passes should be defensed more readily. Alternatively though, the opposite may be true if a defense is atypically aggressive. Summarily, I expect that PD occur more frequently on passing downs. Of course, we will use proportions (e.g., passing down PD/passing down PA) to mitigate the greater quantity of passing known to occur on passing downs. 

So, to start, I randomly selected 27 D1 games from 2014.¹ Play-by-play data of some games required considerable manipulations to fit in the MS Excel parsing system I devised; those were excluded. The sample was too small in my opinion—14 games—but then I remembered the publicly available data from the Football Study Hall 2013 Charting Project; much thanks is due.  It contains play-by-play data from 202 games. I combined that with my comparatively minute data set and computed PD proportions for passing and standard downs for each team in each game.

Non-normal distributions of the standing and passing down PD proportions obligate the use of a Mann-Whitney test. This test compares group median instead of average and lacks assumptions of normal distribution which, if violated, could undermine the results when using techniques that compare averages. That is, the proportional PD data are non-normal.

PD were found to occur more frequently on passing downs than on standard downs (U_observed = 77734 < U_critical = 84127, z = 3.74, p < .001, two-tailed). The median proportion of PD for passing downs, .125 (IQR = .161), was greater than that of standard downs, .097 (IQR = .11), a difference of .028 (95% CI ± ~.014). An effect size of .12 indicates a small to medium difference.

The results corresponded to my expectations but we do need to account for garbage time because defenses adjust their protections. Winning offenses may abstain from passing to prevent PD and INTs that stop the clock and forfeit possession, respectively. Winning defenses may become more amenable to completions underneath coverage while preventing large gains (i.e., less likely to attempt PBU or INT). To adjust for garbage time, I computed PD proportions for passing and standard downs occurring outside of garbage time. The results are similar but the difference is less pronounced. The median proportion of PD on passing downs, .118, was greater than that of standard downs, .10, a difference of .018 (z = 3.02, p =.003). An effect size equal to .07 indicates a small but significant difference.

The results support the hypothesis that a greater proportion of passes thrown on passing downs are broken-up or intercepted than on standard downs. But what else can we glean from this data? We can gain insight into team defensive tendencies, that is, how aggressive a team is at defensing passes.

Although voluminous and grand, the FSH data set contains only 202 of the 868 games played in 2014. Thankfully, however, the project was crowd sourced to who?—to fans. It appears several devotees charted all or nearly all (-1 or -2) games of their beloved warriors. Those teams appear in the table below.

Table 1. Sampling NCAA Team Defenses PD% on Passing and Standard Downs, non-Garbage Time

		Passing Downs		Standard Downs		PD%
Team	Games	PA	PD	PA	PD	Passing Downs	Standard Downs	sdPD% / pdPD%
Virginia Tech	12	184	35	154	28	0.190	0.182	0.956
Illinois	10	104	6	164	9	0.058	0.055	0.951
Northwestern	12	153	26	254	36	0.170	0.142	0.834
Oklahoma	13	176	31	211	30	0.176	0.142	0.807
Cincinnati	10	116	20	138	18	0.172	0.130	0.757
Kansas State	11	132	21	184	19	0.159	0.103	0.649
Washington State	11	135	21	232	18	0.156	0.078	0.499

In the rightmost column we find the ratio of PD on standard downs to PD on standard downs. As that value increases, ostensibly, it indicates how aggressively a defense makes plays on balls in the air. That is, teams that defense more passes on standard than on passing downs may be more aggressive. However, this is not granted; it may be that a team's defense faced pass-drunk teams or faced trailing teams passing to score quicker outside of garbage time. Also unaccounted for is the effectiveness of that aggressiveness, Illinois.

An existing metric, PD-to-INC, or PD to incompletions, is used to gauge defenses' aggressiveness defensing the pass. So, I compared sdPD%/pdPD% to PD/INC and that can be viewed in the table below. The stats used in computing the values shown in the table were culled from College Football Stats. We find some nuance there because, with PD/INC Kansas State appears to be the second-most aggressive defense within this small sample. However, in sdPD%/pdPD% K-State falls to fifth in our set.

Table 2. Comparison of sdPD% / pdPD% to PD-to-INC

Team	PD	Att.	Inc.	PD / Inc.	pPD%/sPD%
Northwestern	70	445	179	0.391	0.834
Kansas State	68	458	181	0.376	0.649
Virginia Tech	70	363	188	0.372	0.956
Oklahoma	62	409	184	0.337	0.807
Cincinnati	57	438	184	0.310	0.757
Washington State	48	458	172	0.279	0.499
Illinois	31	354	123	0.252	0.951

So, in conclusion, passes are defensed more frequently on passing downs than on standard downs. Also, I have suggested that the greater the ratio of PD on standard downs to PD on passing downs, the more aggressive a defense. Virginia tech ranks atop my small sample and there is some support for this notion that the Hokies' secondary and coverage backers were a ball-hawking bunch. However, a shortcoming of the ratio: it provides no indication of effective pass defensing. As it is with science though, we must draw conclusions other than a "shortcoming." Considering the complexities inhere in football, it may be that an aggressive secondary may be rendered less effective by an inadequate pass rush or that its aggressiveness exceeds the execution of its coverage personnel, both of which appear to be the case for the 2013 Illini.

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¹ I assigned each 2014 FBS football game a random number between 0 and 999,999. Next, a list of random numbers between 0 and 999,999 were generated until 27 of those corresponded to a 2014 FBS football game. Beforehand, I anticipated using a Washington State game (highest rate of passing in ’14), a Louisiana Tech game (tied for 1st in total interceptions, 7th in pass break ups), and a Louisville game (Gerod Holliman tied NCAA D1 record of 14 INTs). Then I remembered the FSH data set so the U of L game was excluded.