The Divergence Index

1. A pair counts only when we can check that both exchanges are asking the same question. For Kalshi vs Polymarket, Polymarket has to ask whether a named person or team will win a named contest, and Kalshi has to list a who-wins event for that contest and year with exactly one open market carrying that name. For Limitless, the market itself names the Polymarket or Kalshi market it mirrors, and we fetch that market's live book. In both cases, pairs with a bid and ask more than 10 cents apart, contests ending within a few days, and midpoint gaps over 25 points are dropped. The full rules are on the cross-listed pairs page.

2. For each pair we take the absolute difference between the two midpoints (halfway between bid and ask), in points. Pairs that agree stay in the average, so the index reads low when the exchanges agree.

3. Each index is the volume-weighted mean of those gaps, where each pair's weight is the combined traded volume of both markets. The two are kept as separate series rather than blended, because the Kalshi vs Polymarket archive began in August and a Limitless number mixed in would make older days incomparable. Both are recomputed at every site build from the same live data that powers the markets table, and the daily closing values are archived to build the histories above.

The index is a measurement, not advice. A wide gap can be a real mispricing, a difference in contract wording, or a settlement-rule mismatch; read both contracts before trading either side. See also cross-listed pairs and the biggest price moves.