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Prediction Markets

Event Contracts Game

Read an event brief, work out a fair probability, and quote a two-sided market in cents. Noise flow pays you the spread; informed flow punishes a wrong centre.

Settings

Game difficulty

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Number of rounds
6
Time per round
60s

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How to play

Each round is one event contract that pays 100¢ if the event happens and 0¢ if it does not. Four moves, then the market resolves.

  1. 1Read the briefA base rate for the event and, on most rounds, evidence with a likelihood ratio. The truth can sit a few points either side of your update, inside the noise band the brief states, and it widens with difficulty.
  2. 2Update in oddsThe Bayes update in odds form: convert the base rate to odds, multiply by the ratio when the evidence points toward the event (divide when it points against), and convert back to a probability.
  3. 3Quote your marketA bid and an ask in whole cents, inside the round’s maximum width. The midpoint of your quote is your implied probability, and it is what your calibration is scored on.
  4. 4Face the flowNoise traders pay you the spread, but they walk away if you quote too wide. Informed traders know the truth and only deal when your quote leaves them an edge. Tight wins volume and invites the informed; wide is safe and barely trades.
Worked update·base 25%→odds 1:3→evidence 3x for→odds 1:1→fair 50¢, quote 48 / 53
P&L points

What your fills were worth over the round, with your closing position marked at the event’s true probability.

Calibration points

A Brier-based bonus on your mid. Beating a coin flip earns, a mid far from the truth loses, and 50/50 forever earns nothing.

You are marked against fair value, not against the single outcome: the event still resolves and the reveal still shows it, but a well-priced round is worth the same whichever way it lands. Skips and timeouts score zero on both components.

Strategy

Centre your quote on your posterior, not on the base rate: a wrong centre costs you far more than a wrong width. Then pick the width deliberately. The widest quote is not the best one, because most of the room refuses it; the tightest trades constantly but hands the informed their edge. When the noise band leaves you unsure, widen toward the middle of the range rather than shading your mid.