As always, here’s a quick primer on some of the stats I’ll be using this week (feel free to skip this part if you’ve read this column before):
Expected points added (or EPA)
EPA is a metric showing how many expected points a team added on a given play.
Here’s the short version: it captures the true value of a play by accounting for the context in which it occurred.
The longer version?
EPA is calculated by first determining how likely the next score of the game is -- a touchdown, field goal or safety -- based on a variety of factors that include field position, down, yards-to-go, score differential and time remaining in the game.
More precisely, if a team is first-and-one at the opposing team’s goal line, the model will predict an expected points value near seven; it’s very likely that the team will score a touchdown. So if a player in that situation then runs the ball in for a touchdown, the expected points added (EPA) won’t be very large.
On the other hand, if a team is at fourth-and-19 at their own 20-yard line, there could be negative expected points in that situation; it’s more likely the other team will score next. So if a quarterback throws a deep bomb for a touchdown, that play will have a very high EPA value.
A play is a success if it had a positive EPA value. So if a team has a 60% success rate, that means 60% of their plays had a positive EPA. A run of two yards on first-and-10 is often not a success; a team tends to be less likely to score after such a play.
Average depth of target
This metric quantifies how far a quarterback’s average pass is thrown down the field. It is measured vertically (straight north/south) from the line of scrimmage to where the receiver catches (or doesn’t catch) the ball. This metric is a useful way to quantify and compare the gunslingers against the check-down artists.
Win probability model
There’s something very important to note about the win probability model I will be using for this series. It does not take into account the strength of either team in the match-up. Both teams begin the game with a 50% to win. All the model is doing is looking back over the past decade to say historically, given the current score, possession, time remaining, home-field, etc., how often has this team ended up winning. If you want to see an actual look at how likely the Chiefs are to win the game, just check the vegas odds before the game and at halftime.
So, if this model does not perfectly predict who will win, what good is it? Well, it’s an excellent way to contextualize just how important a play was. We can say that before Mahomes did _________, the Chiefs had a 30% chance of winning, and after, they had a 70% chance. Thus, we can say that the amazing thing Mahomes just did added 40% to the Chiefs win probability - also known as Win Probability Added. This makes our model very useful because it treats all teams equally and thus lets us compare plays across games to see which had the biggest influence on the game.
Now that that’s out of the way, let’s get to analyzing Sunday’s game.
Chiefs at Lions, September 29
Let’s take a look at my blood pressure chart — I mean the Chiefs and Lions win probability throughout the game.
I’ve noted the five plays with the biggest impact on the game:
I mean... sheesh. I have to say — it was kind of nice watching a Chiefs game and not have it be decided by halftime. I missed what that felt like. That said, I certainly didn’t miss the deep knots in my stomach when Stafford threw his two Hail Mary chances. While many of these plays are obvious to include here, I think one might have flown a bit under the radar. On the Chiefs’ final drive, even though they’d driven all the way to the red zone, they still had under 50% chance of winning the game.
That is, until Byron Pringle caught a short laser from Mahomes, took three big hits from a trio of hungry lions, secured the football and got a first down. At that point, it was only a matter of time before we punched it in.
It’s time for my favorite aspect of the Chiefs to analyze every week, and every analyst’s dream — Patrick Mahomes.
Let’s look at how this week’s performance stacked up to the rest of his season thus far:
Uh oh... is that actually regression I see?
This was not a good game for Mahomes, and it was his first game of the season in which he dipped below his league-leading EPA per play of ~0.35 from last year. That said, even on an off week where he was visibly frustrated, he still performed better than the NFL average for 2019 so far. If this is his low point, the Chiefs are going to be just fine (but, you already knew that).
There are two main factors that go into being an efficient quarterback: throwing the ball downfield and completing those throws at a high rate.
Which one did Mahomes struggle with against the Lions?
From the graph above, it appears we can cross off depth of target as being the issue for Mahomes on Sunday. He was actually averaging a greater depth of target than in 2019 so far, and a larger portion of his throws were greater than 10 yards downfield.
What about his accuracy? Well, he only completed 57.1% of his passes. Yes, he was throwing the ball deep, which are more difficult throws to complete, but that is still well below his usual completion percentage, which has been around 70% so far this year.
Now, one particular play from Mahomes this game stood out to me.
On fourth-and-8 on the final drive, Mahomes scrambled 15 yards for a first down. And I’ll be honest — I expected him to make a (not scramble, necessarily, but definitely to move the chains and keep the ball alive). I’ve just grown accustomed to Mahomes pulling something out of his hat on fourth down. So, as us nerds do, I wanted to analyze just how good Mahomes has been on fourth down in his still juvenile career.
Just... wow. This chart includes all quarterback rushes and dropbacks on fourth down since 2009, for all quarterbacks with at least 10 such attempts. Advanced metrics like success rate can feel complicated, but on fourth down, they become fairly straightforward: a play is a success if the fourth down is converted.
Mahomes is converting fourth downs at a higher rate than any quarterback over the past decade. In fact, I even looked at running backs, and only Matt Forte and LeSean McCoy have a higher success rate on fourth downs since 2009. The statistician in me would be upset if I did not point out that Mahomes only has 10 fourth-down attempts here.
We can expect this number to go down a bit as he gets more chances (Tom Brady has had 104 fourth-down conversion attempts since 2009, for example). However, even among all the other quarterbacks with 10 to 20 attempts, not one has come even close to matching this number.
So it looks like I was justified in thinking Mahomes was going to convert here.
Chiefs receiving corps
How did the men on the receiving end of Mahomes’ less-than-stellar performance do?
The receivers on this chart are ordered by how many times they were targeted, from top to bottom. Demarcus Robinson is likely not too happy with his sub-40 yard performance, as he was outproduced by Deon Yelder despite receiving seven more targets.
Missing from the Chiefs’ passing attack the past few weeks has been yards after the catch. Other than the running backs and Yelder, you’ll notice most of these bars are primarily red — meaning the ball traveled the majority of this receiving yards in the air. This will be interesting to keep an eye on. While it’s encouraging the Chiefs can continue to win without the explosive plays and missed tackles, it sure would be nice to have Watkins, Robinson or Kelce blow the top off the coverage or make a guy miss and take it all the way.
Chiefs rushing attack
Instead of just breaking down the running game by looking at how each running back performed, let’s also look at where they performed. Here’s is the Chiefs’ running efficiency against Detroit by run location.
As you can see, the Chiefs ran far better to Mitchell Schwartz’ side of the field and straight up the gut than they did to Cam Erving’s side. In fact, of the eight plays ran to the left (four between the tackles and four to the outside), only one was successful.
Let’s hope Eric Fisher gets back soon.
How’d each back perform?
Running Backs vs Detroit
Darrel Williams on Sunday was one of the rare examples where EPA and success rate are wildly different. Because Darrel converted short-yardage plays when he needed to (including the game-winning touchdown in the fourth), he was “successful” on half his plays, despite the other half being very, very bad from an efficiency metric.
McCoy, on the other hand, had a positive EPA per play, which he has managed to do almost every week so far this year. This is no small feat for a running back — the average so far in 2019 is -0.08 EPA per play, and McCoy is sixth in average EPA of all backs with at least 20 attempts this year.
How often did the Chiefs’ pass?
When analyzing a team’s play-calling tendencies, it is important to separate early-down, neutral game-script plays from the rest of the game. Why? Well, any team that is winning by a large margin is going to run the clock out, and any team that is way behind is going to pass. Furthermore, third and fourth-down play-calling is largely dictated by the yards needed to gain the first down.
So instead of looking at run versus pass percentages over the course of the whole game, we only look at first and second down when the Chiefs’ win probability was between 20% and 80%.
In these situations against Detroit, the Chiefs passed the ball 72.2% of the time — 10% higher than against Oakland or Baltimore. This is really encouraging, as even in a down week for Mahomes the Chiefs average far more EPA throwing the ball than running.
How’d the other side of the ball fare?
Chiefs Defense vs. Detroit
|Play Type||EPA/Play||Success Rate||Attempts||Tackled For Loss / QB Hits|
|Play Type||EPA/Play||Success Rate||Attempts||Tackled For Loss / QB Hits|
The past couple of weeks, the Chiefs’ pass defense had actually looked far better than the run defense, with both Derek Carr and Lamar Jackson averaging negative EPA per play. This was not the case this week, as Stafford seemed to shred the defense at will. In fact, according to Next Gen Stats, Matthew Stafford had the fourth-highest completion percentage over expected—this is a metric which uses tracking data to estimate how likely a pass is to be completed given its location on the field and receiver separation (among some other factors) and compares it to the actual completion rate measured.
Stafford also had the second-lowest expected completion percentage. This means that he was completing passes that were thrown deep and into tight coverage. This measure backs up what we saw on the field Sunday — the Chiefs’ cornerbacks always seem to be in the right position, but just can’t get their head around or get a hand up to stop the ball being caught. Let’s hope veteran Morris Claiborne can change that.
The Chiefs also managed to get three tackles for loss and five quarterback hits — both decent numbers.
That’s it for Week 4 of Stacking the Box Score.
Leave a comment or reach out on Twitter (@chiefsanalytics) if you have questions or would like to see something new for next week. Thanks!