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The great statistician, Hans Rosling, died in February last year. Throughout his life Hans used data to explain how the world was changing – and often improving – and he would challenge people to examine their own preconceptions and ignorance. Before he became ill, Hans had started working on a book about these questions and what they reveal about the mental biases that tend to lead us astray. Tim Harford speaks to his son Ola and daughter in law Anna who worked on the book with him.