luckadjusted

Week 1 2021 · September 12, 2021

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

49ers at Lions, Week 1 2021: who deserved to win?

The 49ers beat the Lions 41–33, but it was a coin flip: replayed with the way both teams played, the 49ers win 6 of 10. Luck ran their way: 3rd & 4th downs alone were worth 1.7 points to the 49ers.

The 49ers won on an off day: they played far below what the matchup promised. In a fresh game against the Lions they would win 88% of the time; with this day’s form, 57%. The numbers

Before kickoff the 49ers were 76% favorites. The biggest swing (Q2, 1:20 left): the Lions were stopped on 3rd & 8, worth 11 points of win probability to the 49ers.

Final

San Francisco 49ers 41
Detroit Lions 33

September 12, 2021 · Ford Field · Closing line: 49ers −9.5

Luck index +43 49ers lucky · −43 Lions

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. 49ers win 6, Lions win 4.

  1. 1 SF 30 DET 3
  2. 2 SF 30 DET 10
  3. 3 SF 37 DET 23
  4. 4 SF 43 DET 34
  5. 5 SF 33 DET 27
  6. 6 SF 22 DET 20
  7. 7 SF 20 DET 23
  8. 8 SF 21 DET 28
  9. 9 SF 14 DET 26
  10. 10 SF 21 DET 43

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.

The 49ers won on an off day

They played far below what the matchup promised. In a fresh game against the Lions they would win 88% of the time; with this day’s form, 57%.

31 points apart for the 49ers: the gap is how differently this game went from what the two teams usually are.

UnitThis gameUsual levelBetter or worse than usual
49ers offenseEPA per play gained +0.95 +0.12 better by 0.83
49ers defenseEPA per play allowed −0.14 +0.09 better by 0.23
Lions offenseEPA per play gained −0.14 +0.14 worse by 0.28
Lions defenseEPA per play allowed +0.95 +0.22 worse by 0.73

EPA (expected points added) per play on competitive snaps, turnovers left out because they are valued as luck. Usual level: the team’s earlier games this season plus half of last season, pulled toward the league average. One team’s defense and the other team’s offense are the same snaps, so one game can’t fully say whose day it was; it shows how far this one was from both teams’ norms.

How 4,000 replays ended

49ers win 57% of them, Lions 43%, counting the 2% still level after regulation as half each. The line is what actually happened: 49ers by 8.

Show as a table
Final margin in the replaysShare of replays
49ers by 17+17.5%
49ers by 9–1615.8%
49ers by 1–822.8%
Level after regulation2.4%
Lions by 1–818.8%
Lions by 9–1612.7%
Lions by 17+10.1%

Where the luck was

Take the luck out

The final margin was 49ers by 8. Remove the luck, one kind at a time, and it becomes 49ers by 9.6.

Each row takes one kind of luck out of the margin. Color shows which team it helped. Luck shares are the fitted ones (how much of each component is luck).

The kicks

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

  • A. Seibert, 51 yds missed · 70% expected
  • A. Seibert, 49 yds made · 72% expected
  • R. Gould, 40 yds made · 88% expected
  • R. Gould, 52 yds missed · 67% expected
  • R. Gould, 52 yds made · 67% expected
Bar: expected chance of a make. Filled dot: made. Open dot: missed or blocked.

When it happened

Lions’ chance of winning

As low as 0% and as high as 65%. 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 Lions were ahead in it, below while the 49ers were.
#WhenWhat happenedWin probability forNext drive
1 Q1 12:06 The Lions were stopped on 4th & 23rd & 4th downs · 61% expected +7% SF DET: Missed field goal
2 Q1 12:01 The 49ers fumbled and the Lions recoveredFumble bounces · loose balls are close to 50/50 +8% DET SF: Touchdown
3 Q1 2:37 The Lions converted 3rd & 83rd & 4th downs · 32% expected +7% DET DET: Touchdown
4 Q1 0:54 The Lions were stopped on 3rd & 23rd & 4th downs · 61% expected +6% SF DET: Touchdown
5 Q1 0:10 The Lions converted 4th & 13rd & 4th downs · 74% expected +10% DET DET: Touchdown
6 Q2 13:33 The Lions converted 3rd & 63rd & 4th downs · 30% expected +9% DET DET: Touchdown
7 Q2 1:20 The Lions were stopped on 3rd & 83rd & 4th downs · 32% expectedReturn touchdown by the 49ersSpecial teams +11% SF DET: Punt
8 Q4 0:58 The 49ers were stopped on 3rd & 133rd & 4th downs · 24% expectedThe 49ers fumbled and the Lions recoveredFumble bounces · loose balls are close to 50/50 +8% DET SF: End of game

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.

  • Taylor Decker (DET T), out (IR) · 1.1 pts
  • Tyrell Crosby (DET OL), out (IR) · 0.4 pts
  • Richie James (SF WR), out (IR) · 0.4 pts
  • Tarvarius Moore (SF DB), out (IR) · 0.3 pts
  • Javon Kinlaw (SF DT), out · 0.3 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

49ers 91% · Lions 9%

Final margin
49ers by 8
Luck in points
1.6 for the Lions
Luck-adjusted margin
49ers by 9.6
Replay with same day's form
49ers 57% · Lions 43% unchanged: why
How much of each is luck?

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

What counts

We tested momentum on 19,000 lucky breaks and found none, so it's off by default. 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 · 49ers · Lions

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