On 17 July 2026, a trader bought 115.07 shares on Polymarket for 22 cents each. Both the public trade record and the on-chain settlement recorded it as a buy. Lee-Ready, a standard algorithm used in financial research, got it backwards and treated the purchase as a sale, overstating its full effective spread by four cents per share. We examine this trade in our new paper, Measuring Trade Direction in a Prediction Market, a follow-up to our earlier study of Polymarket’s order book. We wanted to know how reliably we could tell whether a buyer or seller started a trade, and...