Methodology

ChanceIndex reports probabilities derived from prediction markets — venues where traders buy and sell contracts that pay out based on the outcome of real-world events. The price of a contract is, in effect, the market's estimate of how likely that outcome is.

How a probability is calculated

Each market contract settles at $1 if its outcome happens and $0 if it does not. A contract trading at 62 cents therefore implies roughly a 62% probability. We read that price directly from the source platform and present it as a percentage. We do not adjust, smooth, or model the number ourselves — the figure you see is the market price.

Sources

• Polymarket — via its public Gamma and CLOB APIs.
• Kalshi — a US-regulated exchange, for cross-platform comparison.

How we match questions across platforms

Polymarket and Kalshi word the same real-world question differently. We pair them automatically by comparing the title text and requiring the two markets to settle within a few days of each other, then keep only near-identical matches. When a pair is found, the event page shows both platforms' prices side by side, with the spread between them. Matching is conservative by design: we would rather show one venue than link two questions that only look alike.

How often data updates

We refresh active markets every few minutes and record every reading, which is what builds the probability-history charts. The "updated N minutes ago" note on each page reflects the most recent reading we hold.

Limitations

• Markets can be thin — a low-volume market's price may move on small trades.
• Prices reflect trader expectations, which can be wrong; they are not guarantees.
• Newly opened markets show a note until enough history has accumulated to chart.

ChanceIndex is an informational resource. It is not financial advice and is not a bookmaker. See our affiliate disclosure and about page for more.