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Stacking the Box Score: What Damien Williams’ 84-yard touchdown meant to Chiefs’ chances vs. Chargers

Constant drama gave the Chiefs the win — and a bye in the playoffs.

NFL: Los Angeles Chargers at Kansas City Chiefs Denny Medley-USA TODAY Sports

Welcome to this week’s Stacking the Box Score, where I’ll be quantifying what went down between the Kansas City Chiefs in their most recent game — in this case, a 31-21 win against the Los Angeles Chargers.

As always, here’s a quick primer on some of the stats and data I’ll be using. If you already feel comfortable with these metrics, skip the link and keep reading!

Chiefs vs. Chargers, December 29th

Win probability

There were a lot of high-impact plays in this game. In fact, there were nine plays that swung the win probability more than 10% for either team. Five of these plays helped the Chiefs’ probability to win, while four helped the Chargers’ chances.

That’s a lot of drama for one game — aside from the additional drama provided by the one between the New England Patriots and Miami Dolphins that Chiefs fans were also following.

As measured by these big plays, just how exciting was this game?

Well, it had more big plays than any Chiefs game going all the way back to the AFC Championship game against the Patriots in January, which had 10. Fortunately for the Chiefs, the season finale had a happier ending.

One of the biggest plays against Los Angeles was the 84-yard touchdown run by Damien Williams in the third quarter.

The Chiefs’ rushing attack hasn’t been one of its strengths this season. In fact, from a perspective of expected points and win probability, running the ball has often been a net negative for the Chiefs. This game, however, was the exception — and majority of it came from this monster run, which improved Kansas City’s win probability by 23.3%.

Let’s take a closer look at how incredible this play really was:

This image of the tracking data provided by Next Gen Stats shows where each player was located as Williams crossed the line of scrimmage. If that’s what you know about it, how likely would you think it is that the play could gain even five yards — much less 84 yards?

I don’t have the raw tracking data (though you can see the play develop here using the cool NGS moving dots animation), but from my experience competing in the NFL’s Big Data Bowl (a competition to predict how many yards a running play will gain using a snapshot of the tracking data when the running back receives the handoff), I can tell you that the likelihood of this play gaining any more than five yards is incredibly small. My own estimate would be less than 5%.

So this play was incredibly unlikely — and deeply impactful, creating 6.9 Expected Points Added (EPA). In other words, given the field position, down and distance, an average NFL team would go on to score 0.1 points on that drive. Williams turned that play into seven points, adding 6.9 to the team’s EPA.

As we see here, in terms of EPA, the play was the fifth-best run for the Chiefs in the last decade — and fourth-best by Win Probability Added (WPA). There’s no denying it was a huge play.

For those interested, the top run on this chart is also by Williams — his 91-yard scamper for a touchdown against the Minnesota Vikings in Week 9. The next two are long Jamaal Charles touchdown runs against the New Orleans Saints and Indianapolis Colts in 2012. The fourth is a direct snap to Albert Wilson on a fake punt against the Atlanta Falcons in 2016.

While it is unlikely the Chiefs’ rushing attack will feature many 84-yard runs in the postseason, it would certainly be nice if Williams could break off a few big ones. Otherwise, we’re likely to see more of the same — average runs that hurt (rather than help) the Chiefs’ chance of scoring.

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