‘In these democratic days, any investigation into the trustworthiness and peculiarities of popular judgments is of interest.” So begins an article entitled Vox Populi, which is not about Donald Trump but was published in 1907 by Francis Galton, a pioneer of statistics, by then 85 years old. He had analysed the results of a sweepstake competition held at the West of England Fat Stock and Poultry Exhibition in Plymouth.

An ox was on display. Visitors could buy a postcard for sixpence and write their guess as to the weight of the ox, once slaughtered and dressed. Of 800 cards filled out, Galton rejected 13 as illegible and averaged the rest. The arithmetic mean of the 787 guesses came to 1,197lb. The true dressed weight