On the eve of the UK's general election, Tim Harford takes a look at what polling data can tell us about predicting elections. Is the number of Catholic nuns on the up? What's the long-term trend – are more women becoming nuns in the UK? Tim Harford looks at the figures. Plus, Matt Parker the stand-up mathematician is invited back to the programme to respond to a listener's query about his theory on the best way to find a life partner.