On June 23, 2016, the UK held a referendum: remain in the EU, or leave. Betting markets gave Remain an 85–90% probability. Pollsters predicted Remain. The UK government had no contingency plan for Brexit, and the prime minister had effectively staked his political career on an outcome he himself could not predict with certainty. Brexit won. 51.9% to 48.1%. The pound plunged 10% overnight, its largest single-day drop since 1985. David Cameron resigned the following morning. Global markets lost $2 trillion in value within two days. And Britain spent the next four years trying to figure out what “Brexit” actually meant. The data telling a different story was already there. Online polls had consistently been closer to the truth than telephone polls. The turnout models had never been properly validated against a nationwide referendum. And anti-immigration sentiment in northern England had been systematically underestimated. Betting odds were anchored to the same telephone polls that were later shown to be wrong. No one stress-tested the consensus. An 85% probability became a “fact,” and every plan was built around it. ☘️ Now imagine if the UK government had used a system like Yarrow before the vote. The system receives one question: “Will the UK vote to leave the EU?” Yarrow has independent analysts approach the question from different bodies of evidence: polling data using two different methodologies, regional turnout models, historical precedents from referendums versus elections, and sentiment data from regions that conventional polling struggled to reach. Three analysts flag online polls showing Leave ahead. Two build turnout scenarios in which older, non-urban voters who leaned toward Leave turn out at higher rates than assumed by the polling models. One points out that betting markets are pricing the same telephone polls—not independent information. The system ultimately returns: Remain: 58%, Leave: 42% And it adds: The 85% consensus probability for Remain depends heavily on a telephone-polling methodology that had never been validated in a nationwide referendum. Cameron might still have held the referendum.But the Treasury might have had contingency plans ready. The Bank of England could have positioned itself in advance. And perhaps $2 trillion would not have vanished from global markets within 48 hours simply because everyone was blindsided by an outcome that, if someone had looked at the full evidence base, was clearly possible. The most expensive predictions aren't the ones that turn out to be wrong. They're the ones that are wrong—and that nobody prepared for.