In September, I started a post by asking you to make this assumption:

Assume that it is within a quarterback’s control as to whether or not he throws a completed pass on any given pass attempt. However, if he throws an incomplete pass, then he has no control over whether or not that pass is intercepted.

If that assumption is true, that would mean all incomplete pass attempts could be labeled as “passes in play” for the defense to intercept. Therefore, a quarterback’s average number of “Picks On Passes In Play” (or POPIP) — that is, the number of interceptions per incomplete pass he throws — is out of his control.

After doing the legwork to test that assumption, I reached two conclusions. One, interception rate is just really random, and predicting it is a fool’s errand. Two, using a normalized INT rate — essentially replacing a quarterback’s number of interceptions per incomplete pass with the league average number of interceptions per incomplete pass — was a slightly better predictor of future INT rate than actual INT rate. It’s not a slam dunk, but there is some merit to using POPIP, because completion percentage, on average, is a better predictor of future INT rate than current INT rate.

So, why am I bringing this up today, at the start of Super Bowl week? Take a look at where Sunday’s starting quarterbacks ranked this year in POPIP (playoff statistics included, minimum 250 pass attempts):

Everything about New England’s offense was an outlier this year, so I’m never surprised to see Brady at the top of any list. But what does it mean that Flacco and Kaepernick come in at 2nd and 3rd in (fewest) interceptions per incomplete pass? Well, the initial reaction is that perhaps both quarterbacks were really lucky, particularly Flacco. Kaepernick ranked 11th in completion percentage and finished 4th in interception rate, thanks to his great POPIP. Meanwhile, Flacco ranked 21st in completion percentage and finished 5th in interception rate! That’s a particularly drastic split, although not necessarily one that places him in great company.

Things get even more interesting when you consider the types of throws each quarterback is making. In my second POPIP post, Mike Clay helped me quantify the relationship between interception rate and the length or depth of a throw. Look at the NFL averages below:

On throws behind the line of scrimmage, completion percentage is very high and interception rate is very low; as pass attempts get progressively farther away from the line of scrimmage, the interception rate increases and the completion percentage decreases.

That all makes sense, until we get to this next bit of information, courtesy of the ESPN Stats and Information department: ^[1]Via e-mail, ESPN also added this: Including the postseason, Kaepernick has the best completion percentage (52.6%), yards per attempt (17.0) and QBR (99.7) on throws 20+ yards downfield.

Including the postseason, Joe Flacco and Colin Kaepernick are #3 and #4 respectively in average length of throw. Flacco’s average pass travels 9.93 yards in the air, Kaepernick’s 9.66. Jay Cutler and Andrew Luck are #1 and #2.

So Joe Flacco throws a ton of deep passes. And his completion percentage is pretty low. No issues there. The weird part is that his interception rate is extremely low. It would be pretty simple for me to end this post with the line, “Joe Flacco has had a really low interception rate this year, despite being both inaccurate and a risky passer (as measured by his average length of throw). You can imagine how sustainable that is.”

Except here’s the weird thing: Flacco’s had a really good interception rate most of his career. ^[2]I’m not really sure what to do with this information, but it probably isn’t irrelevant. Flacco threw ten interceptions this year, and I checked the official play-by-play logs to see how … Continue reading It is, of course, possible that Flacco has simply been Wyatt Earp, and his POPIP will regress to the league average in the future. But that’s not the only possibility.

Commenter Red brought up a good point in the original POPIP post:

Could it be that accuracy and decision making are two independent components of QB play? Brett Favre is probably the best example of this. He had several seasons in which he completed over 65% of his passes, yet still had awful INT % in those years (2003 and 2008 come to mind). Favre was always known as a great thrower, but a poor decision maker, which is backed up by the discrepancy between his completion % and INT/INC %.

I suppose that would make Flacco a good decision maker but an inaccurate quarterback? I’m not sure if those are the elements that lead to the fifth best interception rate in the NFL, but it’s worth pondering. Roman Gabriel, Doug Williams, Neil O’Donnell, Neil Lomax, Roger Staubach, and Donovan McNabb threw far fewer interceptions than you would predict based on their completion percentages. Subjectively, Flacco doesn’t really remind me of a McNabb or an O’Donnell, a player who works well within the system and doesn’t take many chances. Joe Montana, Mark Brunell, Ken Anderson and Neil Lomax also had pretty low POPIP ratios, too, although I’m not sure if there’s a common thread among those quarterbacks, either.

So, is Flacco lucky, or is he the type of quarterback who will always have a high average depth of throw, a low completion percentage, and a low interception rate? And if so, why is that the case (perhaps because he’s usually playing in favorable game scripts)? My intuition is that he’s lucky, although he’s been lucky for quite a few years now. If that’s the case, you never know when Lady Luck will turn on you, and Super Bowl Sunday would be as likely a day as any other. But it’s worth pondering whether there is a legitimate reason Flacco’s been able to keep his interception rate so low.

[Update: I e-mailed with Mike Clay after publishing this post, and Mike showed me some numbers indicating that Flacco historically has had a really low interception rate on deep passes. In the second footnote to this post, I noted that only 2 of his 148 passes that were labeled “deep” were intercepted. After Mike pointed out his historical data, that made me take another look at the 2012 numbers. I think deep means greater than or equal to 15 yards, so based on the aDOT/INT table, you would expect 8-12 interceptions on so many deep throws. Considering that Flacco threw only two, and has a history of not throwing interceptions on deep passes, perhaps there’s something there.

On the other hand, I hesitate to give him too much credit, because I would think “trying not to throw interceptions when you go deep” isn’t a skill that only Joe Flacco has acquired. Part of it may also be related to the game script idea hinted at above, in that more of Flacco’s deep passes may come when his team is winning and he doesn’t need to be as risky. All quarterbacks throw more interceptions in losses, but he threw 6 INTs in 5 losses (ignoring the CIN game) and only 4 in 10 wins (13, if you count the post-season). Essentially, it may be that Flacco is throwing passes like he’s losing but since he’s winning, he can be more conservative with them. I’m also not sure how that impacts how we would predict his future interception rate: if his team is trailing, he may revert to the league average in that metric, but if his team continues to be a Super Bowl contender, maybe it won’t.]