Special Note

Advanced NFL Stats is now Advanced Football Analytics. Sorry for the inconvenience and broken links, but the change was long overdue. Although the old address forwards here, you'll likely need to to update your RSS subscription and bookmarks.


Two-Point Conversion in the KC-DEN game

With 7:15 left in the 4th quarter against DEN, KC's Knile Davis ran for a 4-yard TD, narrowing the Broncos' lead 21-16 pending the extra point or two-point conversion. Andy Reid elected for the extra point, and following the kick the Chiefs trailed by 4 points rather than 3 or 5 points resulting from a two-point try.

NFL coaches typically adhere to what's known as the Vermeil Chart for making two-point decisions. The chart was created by Dick Vermeil when he was offensive coordinator for UCLA over 40 years ago. It's a very simple chart that simply looks at score difference prior to any conversion attempt and does not consider time remaining, with one caveat. It applies only when the coach expects there to be three or fewer (meaningful) possessions left in the game.

With just over 7 minutes to play, there could be three possessions at most left, especially considering that at least one of those possessions would need to be a KC scoring drive for any of this to matter. (In actuality, there were only two possessions left, one for each team.) Even the tried-and-true Vermeil chart says go for two when trailing by 5. But it's not the 1970s any more and this isn't college ball, so let's apply the numbers and create a better way of analyzing go-for-two decisions.

Except for rare exceptions I've resisted analyzing two-point conversion decisions with the Win Probability model because, as will become apparent, the analysis is particularly susceptible to noise. Now that we've got the new model, noise is extremely low, and I'm confident the model is more than up to the task.

First, let's walk through the possibilities for KC intuitively. If KC fails to score again or DEN gets a TD, none of this matters. Otherwise:

Chiefs Crawling Drive, Come Away With Nothing

The Chiefs lost to the Broncos 24-17 on Sunday and had a chance to at least tie the game at the very end. Kansas City kept Peyton Manning off the field for an enormous chunk of the second half. The Broncos offense had only two drives after halftime (not including the final kneel down), one for a punt, one for a field goal, totaling just 8:51 in possession. The longest drive came from the Chiefs at the very start of the second half, where they ran 23 plays, taking 10 minutes off the clock... and ultimately missed a field goal. This got me thinking, how does drive length (in minutes) affect the probability of a team scoring?

First, here's a look at the ridiculous drive using our Markov model:

SOE: Weekly Game Probabilities

Weekly game probabilities for week 2 are now up at Sports on Earth. Probabilities are a combination of the pre-season team strength estimates with a small dose of stats from week 1.

Nick Foles and Interception Index Regression

Nick Foles and Josh McCown were two of last season's most pleasant surprises, emerging from obscurity to post two of league's most efficient seasons.  Both finished in the top 3 for Expected Points Added per Play, largely in part because the two combined to throw just three interceptions.

With one week of the 2014 season in the books,  Foles and McCown have already matched that combined total.  While everyone should have expected both to regress from their remarkably turnover-free 2013 seasons, that does not tell us how far each should regress based on historical norms.

Podcast Episode 28 - Chase Stuart

Chase Stuart rejoins the show to break down the surprising week one winners and losers. Chase shares his observations from the Jets home opener and explains some of the weekend's more intriguing game scripts. He also examines the data on exactly how important week one results are in predicting the season while looking ahead to the most intriguing week two match-ups.

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Simulating the Saints-Falcons Endgame

I was asked yesterday about the end of regulation of the Saints-Falcons game. With about a minute and a half remaining, NO was down by 4 but had a 1st & goal at the 1. With 2 timeouts left, should ATL have allowed the touchdown intentionally?

I previously examined intentional touchdown scenarios, but only considered situations when the offense was within 3 points. In this case NO needed a TD, which--needless to say--makes a big difference. Yet, because NO was on the 1, perhaps the go-ahead score was so likely that ATL would be better off down 3 with the ball than up 4 backed-up against their goal line.

This is a really, really hard analysis. There's a lot of what-ifs: What if NO scores on 1st down anyway? What if they don't score on 1st but on 2nd down? On 3rd down? On 4th down? Or what if they throw the ball? What if they stop the clock somehow, or commit a penalty? How likely is a turnover on each successive down? You can see that the situation quickly becomes an almost intractable problem without excessive assumptions.

That's where the WOPR comes in. The WOPR is the new game simulation model created this past off-season, designed and calibrated specifically for in-game analytics. It simulates a game from any starting point, play by play, yard by yard, and second by second. Play outcomes are randomly drawn from empirical distributions of actual plays that occurred in similar circumstances.

If you're not familiar with how simulation models work, you're probably wondering So what? Dude, I can put my Madden on auto-play and do the same thing. Who cares who wins a dumb make-believe game? 

Analyzing Replay Challenges

The new WP model allows some nifty new applications. One of the more notable improvements is the consideration of timeouts. That, together with enhanced accuracy and precision allow us to analyze replay challenge decisions. Here at AFA, we've tinkered with replay analysis before, and we've estimated the implicit value of a timeout based on how and when coaches challenge plays. But without a way to directly measure the value of a timeout the analysis was only an exercise.

Most challenges are now replay assistant challenges--the automatic reviews for all scores and turnovers, plus particular plays inside two minutes of each half. Still, there are plenty of opportunities for coaches to challenge a call each week.

The cost of a challenge is two-fold. First, the coach (probably) loses one of his two challenges for the game. (He can recover one if he wins both challenges in a game.) Second, an unsuccessful challenge results in a charged timeout. The value of the first cost would be very hard to estimate, but thankfully the event that a coach runs out of challenges AND needs to use a third is exceptionally rare. I can't find even a single example since the automatic replay rules went into effect.

So I'm going to set that consideration aside for now. In the future, I may try to put a value on it, particularly if a coach had already used one challenge. But even then it would be very small and would diminish to zero as the game progresses toward its final 2 minutes. In any case, all the coaches challenges from this week were first challenges, and none represented the final team timeout, so we're in safe waters for now.

Every replay situation is unique. We can't quantify the probability that a particular play will be overturned statistically, but we can determine the breakeven probability of success for a challenge to be worthwhile for any situation. If a coach believes the chance of overturning the call is above the breakeven level, he should challenge. Below the breakeven level, he should hold onto his red flag.

Eagles Escape Embarrassment

Let my bias not be unknown, I am an Eagles fan. Watching Nick Foles fumble twice, throw an interception, and Chad Henne connect with rookie Allen Hurns twice for touchdowns -- all in the first half -- was one of the more frustrating ways to start the season. The Eagles were lucky to only be down 17-0 at halftime. On the opening drive of the second half, Philly converted twice and Nick Foles connected with Darren Sproles for eight yards on 3rd-and-9, bringing up a 4th-and-1 at the Jaguars 49-yard line.

Chip Kelly is known for his progressive thinking and he didn't hesitate -- calling for a hurry-up, one of the first times the Eagles really played up-tempo in the game. The Jaguars safeties got crossed and a huge gap opened up as Darren Sproles ran untouched for a 49-yard score. The Eagles would not look back and ultimately went on to win "handily," 34-17. Had the Eagles not converted, though, Kelly would likely have been ridiculed for his call as it could have effectively ended the game (dropping Philly's win probability to 5.4%).

Let's look at the fourth down call, taking into consideration the relative strength of the two teams.