Focus groups, along with opinion polls, are vital for political parties and election campaigns. Peter Mandelson discusses introducing them to British politics, Polly Mackenzie explains why the Lib Dems could never afford them, and even Daniel Finkelstein agrees they make a difference.

The political masterminds join Matt Chorley to talk about when to follow the polls and when to ignore them, Liz Truss's Popular Conservatives, and who has met the most prime ministers?

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