If you go to a gathering of 25 or more people, what are the chances one of you has coronavirus? Imagine that you’re planning to hold some sort of gathering or dinner at your home. Take your pick of big festivities - it’s Thanksgiving in the US, we’ve just had Diwali and Christmas is on the horizon. In some places such a gathering is simply illegal anyway. But if it IS legal, is it wise? Professor Joshua Weitz and his team at Georgia Tech in the US have created a tool which allows people in the US and some European countries to select the county they live in, and the size of gathering they are intending on having, and then it calculates the chances that someone at that party, has Covid 19. Presenter: Tim Harford Producer: Charlotte McDonald