What a price on a prediction market actually means
candidatecasino.com converts every venue's price into a probability, corrects for the venue's own margin, and combines venues into one consensus figure — this post explains the three steps in general, without a real market's numbers attached.
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From a price to a probability
A prediction market's price is not itself a probability — it is a number between 0 and 1 that a venue's own convention turns into one. candidatecasino.com's methodology page sets out exactly which figure each of its two sources uses: Polymarket's bid/ask midpoint, Kalshi's last trade. Whichever figure a venue quotes, reading it as "the probability of this outcome" is the first step, and it is a convention, not a law of markets.
Say, purely as an illustration, a market lists three outcomes at implied probabilities of 52%, 33% and 19%. Add those together and you get 104% — one outcome too many, in probability terms. That extra four points is what the rest of this post is about.
Where the extra points come from
The amount by which a market's implied probabilities sum past 100% is called the overround, or "the vig" — short for vigorish, a bookmaker's traditional cut. It exists because a venue prices in its own margin, or because a thin, illiquid market's spread pushes bid and ask apart until the midpoint no longer settles at a clean 100% across every outcome.
The overround is not evidence that one outcome is more likely than the market says — it is a property of the market itself, spread unevenly across whichever outcomes happen to carry it.
De-vigging: scaling back to 1.0
De-vigging removes the overround by scaling every outcome's implied probability down proportionally, so the corrected figures sum to exactly 1.0. In the illustrative example above, each of the three outcomes loses a share of the excess four points in proportion to its own size — the 52% outcome gives up more percentage points than the 19% outcome does, in absolute terms, even though the proportional cut is identical for all three.
Why this matters more than it looks
The size of the correction depends on how much overround a market carries and how many outcomes it lists. A market with two outcomes and a wide spread can carry a much larger overround than a nine-outcome market that is already close to 1.0 — which is why two venues pricing the same real-world question can show different de-vigged figures even when their raw prices look similar. The full methodology page walks through this with real, cited numbers from an actual market; this post is deliberately about the mechanism, not a specific price.
— candidatecasino.com