Galton called this phenomenon ‘regression towards mediocrity’, though today it’s known as regression to the mean. He then realised that the mathematical tools he’d used to track the difference between two expressions of height (that of parent and child) could be tweaked to measure the strength of connection between any two variables, to see if they moved in tandem or not at all. These measurements didn’t have to belong to the same scale, but could encompass any set of figures, connecting anything from temperature and suicide rate to an individual’s beard length and buoyancy. Galton named his
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