Polymarket runs a real-money market on this question. Norynta mirrors it with play credits: it opened at the Polymarket price and settles exactly as that market does. Players and a labelled market tracker move the price from there. Think the crowd is wrong? Trade against it.
This market will resolve to "Up" if the official S&P 500 Index closing price for S&P 500 (SPX) on Monday, October 19, 2026 is higher than the official S&P 500 Index closing price for SPX on the most recent prior trading day. This market will resolve to "Down" if the official S&P 500 Index closing price for S&P 500 (SPX) on Monday, October 19, 2026 is lower than the official S&P 500 Index closing price for SPX on the most recent prior trading day. E.g., ordinarily, a market on Monday would refer to the previous Friday for its most recent closing price, unless that Friday were a market holiday, in which case it would refer to Thursday, or the next most recent trading day. If the two specified closing prices are exactly equal, this market will resolve 50-50. Note that all figures will be rounded to the nearest cent using standard rounding. If SPX does not trade at all during the regular session, the market will resolve 50-50. If either of the relevant days are shortened (for example, due to a market holiday schedule), the official closing price published by S&P 500 Index for that shortened session will still be used for resolution. The full rules are on the Polymarket market. Resolution: this play-money market resolves to the same result as the official Polymarket market settlement, linked as this market's resolution source. Observation time: the source market's final settlement after trading closes at 2026-10-19T20:00:00Z (UTC). If the source market is voided, cancelled, or its result is unavailable or ambiguous, this market is voided and positions are refunded.
Forecasts and play credits have no cash value. At resolution each winning share pays 1 play credit and your calibration record updates.