Last off-season, I produced an exhaustive analysis of fumble recovery rates . With 13 years of play-by-play data at my disposal, I thought it would be worthwhile to take a closer look at interceptions. You can skip to the results section if you like, but let me start with a few disclaimers.

Interceptions are tricky to analyze. Interception rates are very inconsistent from year to year, so much so that completion percentage alone may be a better predictor of future interception rate than actual interception rate. But putting aside randomness, there are two other big factors that determine interception rates: the score in the game and the length of the throw.

Just about every quarterback will throw more interceptions when his team is trailing in the fourth quarter. Situation plays a huge role in football, and that’s true when it comes to interception rates, too. Similarly, quarterbacks are much more likely to be intercepted on deep passes than short ones. One thing I wanted to look at was how much league-wide interception rates varied over a wide range of circumstances.

Unfortunately, there still is a bit of bias in the data. The best quarterbacks are most likely to be winning and the worst quarterbacks are most likely to be losing. That means to the extent that trailing teams throw more interceptions than leading teams, the results are probably slightly overstated. Still, I think getting a sense of the league baseline over hundreds of thousands of throw — even if not evenly distributed — can be a useful exercise.

The Results

From 2000 to 2012, there were 6,689 interceptions thrown. Here’s the breakdown with respect to the points differential (i.e., points scored minus points allowed) for the offensive team immediately before the interception. For example, teams trailing by more than four touchdowns have thrown 110 interceptions over the last 13 regular seasons. That accounts for 1.6% of all interceptions thrown over that time period:



Category INT Perc (%) Trail > 28 points 110 1.6 Trail 22-28 pts 257 3.8 Trail 15-21 pts 619 9.3 Trail 8-14 pts 1161 17.4 Trail 1-7 pts 1874 28.0 Tie Game 1079 16.1 Lead 1-7 pts 978 14.6 Lead 8-14 pts 400 6.0 Lead 15-21 pts 153 2.3 Lead 22+ points 58 0.9 Total 6689 100.0

But looking at interceptions alone isn’t enough: interception rate is the more important stat. This next table shows the average interception rate in each of those categories. Teams trailing by 22 to 28 points have accounted for 2.9% of all passes attempts in the NFL since 2002; but as we saw from the table above, 3.8% of all interceptions occurred in this situation. That’s because the interception rate is pretty high when teams trailed by 22 to 28 points: passers trailing in that range threw one interception every 25 passes:

Category Pass Perc INT INT Rate Trail > 28 points 2857 1.3% 110 3.9% Trail 22-28 pts 6446 2.9% 257 4% Trail 15-21 pts 16274 7.4% 619 3.8% Trail 8-14 pts 32177 14.7% 1161 3.6% Trail 1-7 pts 56021 25.6% 1874 3.3% Tie Game 41317 18.9% 1079 2.6% Lead 1-7 pts 38234 17.5% 978 2.6% Lead 8-14 pts 16324 7.5% 400 2.5% Lead 15-21 pts 6238 2.9% 153 2.5% Lead 22+ points 2862 1.3% 58 2% Total 218750 100% 6689 3.1%

We can also break interceptions down by quarter. Here’s how to read the next table: Teams have thrown 32.8% of all interceptions in the 4th quarter despite “only” 27.6% of all passes coming in that quarter. That’s because passers have a 3.6% interception rate in the fourth quarter.

Quarter INT Perc Pass Perc Rate 1 1199 17.9% 46363 21.2% 2.6% 2 1814 27.1% 62121 28.4% 2.9% 3 1456 21.8% 48846 22.3% 3% 4 2193 32.8% 60409 27.6% 3.6% 5 27 0.4% 1011 0.5% 2.7% 6689 100% 218750 100% 3.1%

I also have data on pass direction — short left, short middle, short right, deep left, deep middle, and deep right — but unfortunately I only have that for half the data set. Still, that gives us over 100,000 passes to analyze. Here, “deep” means anything over 15 yards from the line of scrimmage. And as you can see, “deep middle” is the most dangerous pass a quarterback can attempt. 500 interceptions were thrown with that designation, which accounts for 14.9% of all interceptions that have directions attached to them in my database. That might not sound like a high percentage, but only 5,949 passes — or 5.1% of all passes — were thrown in the deep middle portion of the field. The interception rate on passes 15+ yards down the middle of the field was 8.4%.

Direction INT Perc Pass Perc RATE Short Left 579 17.3% 33111 28.5% 1.7% Short Middle 572 17.1% 22309 19.2% 2.6% Short Right 662 19.8% 38695 33.3% 1.7% Deep Left 479 14.3% 7951 6.8% 6% Deep Middle 500 14.9% 5949 5.1% 8.4% Deep Right 553 16.5% 8283 7.1% 6.7%

Now that we have a sense of the base rates, let’s start combining the data. This next table shows the number of pass attempts (in the left columns) and the interception rate (in the right columns) in each quarter broken down by score. There’s a big difference in fourth quarter interception rates between teams trailing by a touchdown or less and teams leading by a touchdown or less. Teams trailing by 1-7 points threw 16,111 passes in the 4th quarter and had an interception rate of 4.18%. Meanwhile, teams leading by 1-7 points in the 4th quarter threw 6,751 passes and had an interception rate of just 2.53%.

Score 1(Att) 2(Att) 3(Att) 4(Att) 1(INT%) 2(INT%) 3(INT%) 4(INT%) Trail > 28 points 0 137 703 2017 -- 5.11 3.13 4.02 Trail 22-28 pts 9 596 1936 3905 0 5.7 3.93 3.76 Trail 15-21 pts 158 3292 4651 8173 1.27 3.86 3.16 4.2 Trail 8-14 pts 1725 9067 7909 13474 2.96 3.25 3.59 3.94 Trail 1-7 pts 11543 17664 10660 16111 2.86 2.98 3.22 4.18 Tie Game 25249 8389 3391 3339 2.54 2.83 2.42 2.82 Lead 1-7 pts 6857 14966 9645 6751 2.38 2.62 2.6 2.53 Lead 8-14 pts 776 6015 5840 3691 1.29 2.39 2.79 2.25 Lead 15-21 pts 41 1650 2753 1794 4.88 2.73 2.36 2.29 Lead 22+ points 5 345 1358 1154 0 2.03 1.69 2.43

Now, what if we combine the score and quarter data with the information we already have on the distance of the throw? That brings us to the most complicated — but also the coolest — table. This groups all throws by either short or long, but otherwise presents the same data (although as noted earlier, the sample size is half as big as in the other set).

The table shows the interception rate for every quarter/score/distance combination. Each cell shows the interception rate and the number of pass attempts in that situation in parentheses. For example, take a look at the intersection between “Trail by 8-14 points” and “4Q/Deep.” In that situation, there have been 1,398 passes thrown and the interception rate was 9.5%. Meanwhile, look at the “Lead 8-14 points” and “4Q/Short” cell. There were 1,506 short throws in the fourth quarter when a team had a 8-14 point lead, and the interception rate was just 1.3%. Obviously, situation matters significantly, and not all fourth quarter throws are created equally.

Score 1Q/Short 1Q/Deep 2Q/Short 2/Deep 3Q/Short 3Q/Deep 4Q/Short 4Q/Deep Trail > 28 points --% (0) --% (0) 1.2% (82) 13.3% (30) 2.8% (357) 2.9% (68) 2.6% (936) 9% (223) Trail 22-28 pts 0% (6) 0% (3) 3.4% (261) 10.7% (56) 2.5% (954) 10.8% (203) 2.7% (1831) 9.6% (374) Trail 15-21 pts 0% (65) 8.3% (12) 2.3% (1401) 9.4% (340) 2.7% (1943) 5.4% (479) 2.5% (3512) 10.5% (783) Trail 8-14 pts 1.7% (775) 8.6% (187) 1.9% (3900) 6.6% (899) 2.8% (3381) 5.9% (798) 2.1% (5581) 9.5% (1398) Trail 1-7 pts 1.9% (5187) 6.5% (1164) 1.8% (7442) 7.3% (1816) 2% (4452) 7.2% (1143) 2.4% (6404) 9% (1974) Tie Game 1.8% (11042) 6.6% (2238) 1.6% (3591) 6.6% (833) 1.7% (1390) 5.3% (320) 1.8% (1353) 6.4% (346) Lead 1-7 pts 1.8% (3032) 5.8% (692) 1.6% (6324) 4.5% (1567) 1.6% (4261) 6.8% (939) 1.5% (3039) 5% (621) Lead 8-14 pts 1.4% (345) 3.5% (85) 1.4% (2586) 4.8% (650) 1.5% (2474) 5.9% (562) 1.3% (1506) 5.3% (321) Lead 15-21 pts 0% (19) 25% (4) 1.2% (684) 5.9% (169) 1.6% (1267) 4.5% (290) 1.9% (782) 5% (159) Lead 22+ points 0% (4) 0% (1) 1% (197) 4% (50) 1.1% (638) 3.6% (137) 2.2% (593) 5% (101) Total 1.8% (20475) 6.5% (4386) 1.7% (26468) 6.3% (6410) 2% (21117) 6.3% (4939) 2.2% (25537) 8.4% (6300)

This is one of the reasons I find the concept of Game Scripts really important to understand. And it’s another reason not to get too obsessed with last year’s interception numbers. The Chiefs, Cardinals, and Jets all finished in the bottom three in both interceptions and interceptions rate. All three teams acquired new quarterbacks in the offseason, so they won’t represent good case studies. But Kansas City, Arizona, and the Jets were also just bad teams last year, and bad teams tend to throw more interceptions because they’re trailing.

As a group, they threw 60 interceptions on 1,576 passes in 2012, giving them a 3.8% interception rate. But one quarter of those interceptions came in the second half when trailing by between 8 and 21 points. Those three teams threw just 350 passes in those situations, giving them a 4.3% interception rate when passing in the second half and down by 8-21 points. To the extent that parity exists in the NFL, the snowball effect when it comes to interceptions won’t persist from year to year. That means if those teams improve, they’ll be less likely to be in high-risk situations, giving us just another reason why interception rates fluctuate wildly every season.