If it seems the EU referendum debate just involves two politicians shouting contradictory statistics at each other - then we are here to help. In this series, we're giving you a break from the politicians and we're going to try to figure out the truth. Bracing concept, isn't it? We'll be looking at some of the big questions - the cost of being a member, immigration, lawmaking and regulation. But today we're looking at trade. Tim Harford asks if the UK would be better off in or out when it comes to trade with other nations.