Was That August Tear Real? Win Streaks and the "Luck Scenario" Yardstick

At the end of the last post, I left this question.

What about within a season? That team "winning like crazy in August" — is the hot streak real, or is that luck too? A coin comes up heads 7 times in a row; how is a baseball team's August tear any different?

The one-line summary of this post:

Barely different. Over 162 games, even a momentum-free coin runs off 7–8 wins in a row almost every season.

First: how do we even decide if it's real?

Last time we looked at luck between seasons. About 87% of this year's residual vanished by next year — luck regressed to the mean.

Now it's within one season. When a team wins 9 straight in August, we say they "caught fire," they're "on a run." But how would we show that's real? More fundamentally — what does "real" even mean here?

To judge whether a pattern means anything, you first have to know what the data would look like if nothing had happened at all. That "nothing happening" world is your baseline. If the real data is indistinguishable from it, the pattern you saw was an illusion.

Statistics calls this baseline a null model. In this post we build it like this:

Replace a team's season with a biased coin that wins each game with the same fixed probability p. No momentum, no memory. Winning yesterday doesn't make it flip better today. p is the win rate that team actually posted that season.

Let's call this fake coin season the "luck scenario" — "if this had all been luck, here's how the data would look."

Run the luck scenario thousands of times

Take the 2024 Yankees. 94–68, a .580 team, and they won 8 straight in August. "Yankees are red hot," went the headlines.

Now turn the Yankees into the luck scenario: a coin that still wins 58% but has zero momentum. Run a 162-game season with that coin 20,000 times, and count the longest win streak each season. Then ask — what fraction of those momentum-free seasons produce an 8-game streak or longer?

The answer is 58%. A no-momentum coin runs off 8+ wins in more than half its seasons. The Yankees' 8-game streak was something a coin does routinely.

Let's call this number — "the chance the luck scenario produces a streak this long or longer" — the luck %. Higher means an ordinary streak (well explained by luck); lower means a streak that's hard to explain with a coin.

Even all-time great teams look similar. The 2022 Dodgers won 111 games and won 12 straight — and even that 12-game streak has a luck % of 41%. A coin with the same win rate does it 4 times out of 10.

A picture: put the real team and the coin side by side

This time, along with streak length, look at the season's full arc. Compute the last-10-games win rate (wins in the trailing 10) at every game and connect them, and you get a curve that rises and falls all season. An August tear shows up as a stretch where this curve spikes up.

Dodgers 2025 last-10 win% curve wobbling in and out of the luck scenario's 80% band

The blue line is the real team's curve. The gray band is the luck scenario — the range where a same-win-rate, momentum-free coin's 10-game rate sits 80% of the time. Notice the band is flat. A coin has no trend, so its curve wobbles up and down but on average stays at one height.

The real team's blue curve clearly wobbles too — it spikes and sags. But most of that wobble happens inside the gray band. In other words, the peak we called an "August surge" is indistinguishable from the wobble a momentum-free coin produces too. Swap teams on the just-mlb Q5 page and most curves never stray far outside the band.

Checking with numbers: did winning lead to more winning?

Curves are intuitive, but judging by eye can be wrong, so let's check with numbers too. If momentum were real, a team should win more right after a win. So we split win rate by the previous game's result — what statistics calls conditional win%.

90% of longest streaks within luck's 90%; conditional win% .516 after a win vs .485 after a loss; mean runs-test z −0.15

Pooling all 150 team-seasons across 2021–2025:

Win rate after a win: .516 / win rate after a loss: .485. The gap is just 3 points. If momentum were real these should split widely, but they're nearly stuck together. (Even that small gap partly reflects strong teams having both streaks and high win rates.)
The runs test z — which checks whether wins and losses are abnormally clustered — averages −0.15, right around zero. There's nothing you'd call clustering across the season.
And the key number: 90% of the 150 team-seasons' longest streaks fall within the luck scenario's 90% range. Nine times out of ten, even a season's longest streak was a length a coin commonly produces.

So is nothing ever real?

There are. If 90% is luck, 10% isn't. Sort by lowest luck % and the genuine exceptions, the ones a coin can't explain, rise to the top.

Sorted by luck % ascending — Cardinals 2021 (17) and Mariners 2022 (14) rise to the top as "exceptions"

2021 Cardinals — 17 straight (luck 0%). A September 17-game run that put them in the playoffs. A same-win-rate coin barely produces it across 20,000 seasons.
2022 Mariners — 14 straight (luck 2%). The streak that ended a 21-year playoff drought.
2025 Twins — 13 straight (luck 0%). And they were a .436 losing team. A coin almost never produces a 13-game streak for a losing team.

These streaks are statistically unusual events that the coin baseline doesn't explain. The conclusion has the same shape as last post's regression to the mean: most of it is luck, and only the few that pass the baseline are real.

To sum up

To check whether an in-season streak is "real," you first ask what a momentum-free coin (the luck scenario) does at the same win rate — that's the null model
The 2024 Yankees' 8-game streak is luck 58%, even the 2022 Dodgers' 12-gamer is 41% — even great teams' streaks are mostly what a coin does anyway
The 10-game win% curve wobbles but mostly inside the luck band; conditional win% is .516 after a win vs .485 after a loss — barely a split
90% of longest streaks sit within luck's 90%. Only the 10% — the 2021 Cardinals (17), the 2022 Mariners (14) — are real exceptions

Next time

So far we've seen that the swings within a season is mostly luck. Which raises one more question.

Within a season it was luck. But what about the end of a season? A team that finished September on fire — does it carry that form into next April?

Between games within a season there was no memory. But a season's final month — if it reflects a settled roster and called-up prospects — might be a signal for next season. Next time we look at whether late-season performance predicts the next season.

You can play with this analysis on the Q5 page of just-mlb, which I built myself. Swap teams and seasons to see whether the curve escapes the luck band, and sort the all-teams table by luck % to find the exception teams yourself.