luckadjusted
NFL Week 9 · 2026

Week 1 2021 · September 12, 2021

No FTN charting for this game (drops and interception-worthy throws are not modelled).

Browns at Chiefs, Week 1 2021: who deserved to win?

The Chiefs beat the Browns 33–29, and the replays lean their way: with the same day's form, they win 7 of 10.

Before kickoff the Chiefs were 66% favorites. The biggest swing (Q1, 9:18 left): the Browns converted 4th & 3, worth 9 points of win probability to the Browns.

Final

Cleveland Browns 29
Kansas City Chiefs 33

September 12, 2021 · GEHA Field at Arrowhead Stadium · Closing line: Chiefs −5.5

Luck index +29 Chiefs lucky · −29 Browns

Each team’s own plays from this game, resampled into new drives 4,000 times: with the way both teams actually played, how often does this performance win? How the replay works

Ten replays. Browns win 3, Chiefs win 7.

  1. 1 CLE 37 KC 21
  2. 2 CLE 20 KC 12
  3. 3 CLE 33 KC 30
  4. 4 CLE 24 KC 26
  5. 5 CLE 24 KC 29
  6. 6 CLE 27 KC 34
  7. 7 CLE 19 KC 30
  8. 8 CLE 20 KC 34
  9. 9 CLE 24 KC 44
  10. 10 CLE 21 KC 47

Three other ways to ask

Each answers a different question, so none of them replaces the replay above.

The closing betting line, turned into a probability. What everyone knew before the game.

The line plus what this game showed, weighted by how much one game can tell. It stays close to the line because one game says little.

Every kick, fumble bounce, drop, conversion and judgment call redrawn; everything else fixed. The narrowest view of luck.

How 4,000 replays ended

Browns win 29% of them, Chiefs 71%, counting the 4% still level after regulation as half each. The line is what actually happened: Chiefs by 4.

Show as a table
Final margin in the replaysShare of replays
Browns by 17+3.9%
Browns by 9–167.5%
Browns by 1–815.4%
Level after regulation4.0%
Chiefs by 1–824.7%
Chiefs by 9–1622.1%
Chiefs by 17+22.4%

Where the luck was

What the luck was worth

Every lucky and unlucky play, weighted by the moment it happened: its swing in win probability, added up by kind. A missed kick in a decided game counts for nothing; the same kick at the end of a tie game can be most of a win.

Swings add up across a game, so in one that went back and forth they can pass 100%. On the scoreboard the same luck was worth +2.1 points to the Browns: the final margin was Chiefs by 4, without the luck Chiefs by 6.1. Luck shares are the fitted ones (how much of each kind is luck).

The kicks

How likely each kicker was to make that kick (his own record, the distance and the weather), and what happened.

  • H. Butker, 28 yds made · 98% expected
  • H. Butker, 43 yds made · 84% expected
Bar: expected chance of a make. Filled dot: made. Open dot: missed or blocked.

Penalties: Browns 5 for 30 yards, Chiefs 5 for 36 yards

Every accepted flag, in order, and what it did to the helped team’s chance of winning at that moment, including any play it wiped out (points on the scoreboard in grey). A flag in a decided game is worth nothing; one at the turn of a close game can be worth most of a win. Judgment calls count half toward luck; pre-snap fouls, which repeat, don’t count. How calls are valued

WhenFlagWin probability for
Q1 7:53 Encroachment on the Chiefs (Chris Jones), 1 yard · pre-snap +2% CLE 0.0 pts
Q1 3:20 Offensive Holding on the Chiefs, 10 yards, wiping out the play +6% CLE +2.0 pts
Q1 2:56 Defensive Offside on the Browns (Myles Garrett), 5 yards · pre-snap 0% KC +0.2 pts
Q2 10:49 Defensive Offside on the Browns (Jadeveon Clowney), 5 yards · pre-snap +1% KC +0.2 pts
Q2 6:24 Illegal Shift on the Browns, 5 yards, wiping out the play · pre-snap +5% KC +1.6 pts
Q2 3:29 Face Mask on the Chiefs (Derrick Nnadi), 15 yards +4% CLE +1.8 pts
Q2 0:37 Offensive Holding on the Browns, 10 yards +1% KC +0.8 pts
Q3 7:47 False Start on the Chiefs (Tyreek Hill), 5 yards · pre-snap +5% CLE +0.7 pts
Q3 0:53 False Start on the Chiefs, 5 yards · pre-snap +3% CLE +0.8 pts
Q4 7:04 Illegal Shift on the Browns, 5 yards · pre-snap +5% KC +0.9 pts

Browns penalties, every game · Chiefs penalties, every game · All teams, 2021

When it happened

Chiefs’ chance of winning

As low as 13% and as high as 100%. Numbers mark the biggest luck moments, listed below.

nflfastR’s in-game win probability. It ignores the betting line, so it starts near even. Shaded above the middle while the Chiefs were ahead in it, below while the Browns were.
#WhenWhat happenedWin probability forNext drive
1 Q1 9:18 The Browns converted 4th & 33rd & 4th downs · 57% expected +9% CLE CLE: Touchdown
2 Q1 7:53 The Browns’ two-point try was good3rd & 4th downs · 47% expectedEncroachment on the Chiefs (Chris Jones), 1 yardPre-snap flags · 17 flags in 1658 snaps before, position avg 1 per 273 +7% CLE CLE: Touchdown
3 Q1 3:20 Offensive Holding on the Chiefs, 10 yards, wiping out the playJudgment calls +6% CLE CLE: Touchdown
4 Q3 6:03 The Browns fumbled and the Chiefs recoveredFumble bounces · loose balls are close to 50/50 +6% KC CLE: Touchdown
5 Q4 8:50 The Browns were stopped on 3rd & 123rd & 4th downs · 28% expected +6% KC CLE: Turnover on downs
6 Q4 7:08 The Chiefs converted 3rd & 33rd & 4th downs · 50% expected +7% KC KC: Touchdown
7 Q4 5:34 The Browns were stopped on 3rd & 73rd & 4th downs · 38% expected +7% KC CLE: Punt
8 Q4 3:41 The Chiefs were stopped on 3rd & 103rd & 4th downs · 33% expected +6% CLE KC: Punt

Missing players

Key players (usually on the field for half the snaps or more) who missed this game or part of it, valued by position and quality. The same players are missing in every replay, so this counts as luck in points only.

  • Frank Clark (KC DE), out · 0.8 pts
  • Tyrann Mathieu (KC SS), out · 0.8 pts
  • Tyrann Mathieu (KC S), out · 0.8 pts

Adjust the assumptions

Disagree with how much of a fumble bounce or a pass interference call is luck? Change it and the coin-flip replay reruns in your browser.

Only coin-flips re-rolled

Browns 15% · Chiefs 85%

Final margin
Chiefs by 4
Luck in points
2.1 for the Browns
Luck-adjusted margin
Chiefs by 6.1
Replay with same day's form
Browns 29% · Chiefs 71% unchanged: why
How much of each is luck?

0% treats it as skill, 100% as pure luck. The tick is our fit.

What counts

None by default: our tests measured no carryover after a lucky break. Slide it up to count a break for more than its own swing. Why

Flip a play

Force the other outcome, in this game and in every replay: a made kick becomes a miss, a stop becomes a conversion, a flag goes the other way.

About these numbers

Play-by-play and win probability from nflfastR via nflverse; kicks valued against each kicker’s own record; drops and interception-worthy throws from FTN charting (not available for this game); injuries from snap counts and injury reports. No FTN charting for this game (drops and interception-worthy throws are not modelled).

Methodology and validation · All Week 1 2021 games · Browns · Chiefs

More from Week 1 2021