2021 Mid-Season Invitational - Rumble Stage Recap

With the Rumble stage of the 2021 Mid-Season Invitational closed out, it's time for another look at how this tournament has played out. Like the group stage, this started out very favorite-dominated: in the first two days, there was only a single upset (PSG Talon over Cloud9). There were a few notable underdog wins on later days however, which in fact gave PSG the edge over C9 to squeeze their way into the Knockout stage!

Days 1 and 2

Let's take a closer look at the first two days, from the market's perspective (and from the perspective of the underdog, to keep things consistent):

|   day |        team |            opponent | pct_win |   win |
|-------|-------------|---------------------|---------|-------|
| Day 1 |     DWG KIA | Royal Never Give Up |   49.2% | FALSE |
| Day 1 | Pentanet.GG |           MAD Lions |  18.73% | FALSE |
| Day 1 |   PSG Talon |             DWG KIA |  20.81% | FALSE |
| Day 1 |      Cloud9 | Royal Never Give Up |  19.43% | FALSE |
| Day 1 | Pentanet.GG |           PSG Talon |  23.42% | FALSE |
| Day 1 |      Cloud9 |           MAD Lions |  45.62% | FALSE |
| Day 2 | Pentanet.GG | Royal Never Give Up |  12.66% | FALSE |
| Day 2 |      Cloud9 |             DWG KIA |  25.25% | FALSE |
| Day 2 |   MAD Lions | Royal Never Give Up |  23.42% | FALSE |
| Day 2 |   PSG Talon |              Cloud9 |  44.06% |  TRUE |
| Day 2 | Pentanet.GG |             DWG KIA |  15.81% | FALSE |
| Day 2 |   PSG Talon |           MAD Lions |  41.56% | FALSE |

Across 12 games, the only upset was PSG Talon v Cloud9 on Day 2, and PSG wasn't a huge underdog anyway - the market priced them at around a 44% chance to win.

How many upsets would we have expected? Recall again that this is simply the sum of each individual match's expected chance to win (from the perspective of the underdog, since we're trying to find the expected number of upsets). Adding up these probabilities, we should expect, on average, 3.40 games won by the underdog. Just a single win is quite a ways off.

...and the rest of the days

Days 3 through 5 made up for this slow start, with PSG Talon in particular overperforming its expectations and securing its slot in the semifinals. Here are all of the upsets of the Rumble stage:

|   day |        team |            opponent | pct_win |  win |
|-------|-------------|---------------------|---------|------|
| Day 2 |   PSG Talon |              Cloud9 |  44.06% | TRUE |
| Day 3 |   PSG Talon | Royal Never Give Up |   22.1% | TRUE |
| Day 3 |   PSG Talon |           MAD Lions |   42.3% | TRUE |
| Day 4 |      Cloud9 | Royal Never Give Up |  20.81% | TRUE |
| Day 4 | Pentanet.GG |              Cloud9 |  23.42% | TRUE |
| Day 5 |   MAD Lions | Royal Never Give Up |   22.1% | TRUE |
| Day 5 |   PSG Talon |              Cloud9 |  46.93% | TRUE |
| Day 5 |      Cloud9 |           MAD Lions |  48.13% | TRUE |

These aren't all just 45% to win "underdogs" either - RNG managed to get beaten out on ~20% shots three times! They were the clear favorites going into this stage of the tournament, and constituted DWG KIA's only two losses, so any one of these upsets not happening would have secured their top seed.

Cloud9 also managed to lose three games that it should have won, and its big upset against RNG wasn't enough to break into the semifinals.

Playing into the market

Of course, just having some fun upsets isn't enough to make a profit. While people heuristically think of 20% win probabilities as some crazy outside shot, they should happen, well, 1 in 5 times. If the market has properly calculated these odds, the money you lose betting on the underdogs that didn't end up winning should end up more than offsetting the big payouts you get from upsets.

In fact, this is sort of how oddsmakers operate - they're happy to pay you out these rare big wins, as long as you keep coming back and they keep taking their vig. Once again, looks like the market was pretty efficient, as you'd lose money betting on either the favorites or the underdogs exclusively:

Turns out betting on the favorites would actually have been better - the winnings from the underdog side of things were simply not that substantial, especially since four of those upsets really just paid out at roughly even odds. And, of course, earning $7 and then $10 on your $100 bets on RNG over PGG isn't really a path to success either. In fact - market odds on those two matches had PGG at 12.7% and 15.0% chances to win, respectively, which is really not that improbable. Payout compression on the favorite due to the vig strikes again!

Our performance

As mentioned above, the market for these games was very efficient - much more so than usual, in fact. There isn't really a good way to know if this one simulation of reality was just favorable to the market, or if they genuinely set their lines perfectly, so let's just split the difference and call it luck.

In due fashion, we straddled that line almost perfectly:

As always, we almost exclusively bet on underdogs, since our typical view is that the market overvalues favorites. So, when the market manages to value favorites versus underdogs just right, it's hard to make money.

The best way to combat this is to pick your battles wisely - if you're able to bet more money on the matches that you end up winning, that's great - but to do so, the actual outcomes need to more closely align with your predictions of probabilities than the market's. On average they do - which is why our long-term performance is pretty steadily positive (see our Spring 2021 recap for a discussion of this), but as they say, you win some and you lose some.

Semifinals and beyond

With only three matches left in the Mid-Season Invitational, our betting naturally will wind down. These matches are obviously going to be more exciting from the perspective of esports, rooting for your team, tournament hype, and all that jazz, but a quantitative model doesn't really differentiate on those things.

Turns out we agree with the market on the two semifinal matches anyway:

We think DWG KIA is even more favored than the market does, in an unusual turn of events, but the payout is so poor that it's not worth it anyway. Check back in after the 21st to see our prediction on the (very likely) RNG vs DK final!

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