The trick that lies at the heart of this kind of automated, algorithmic detective work is correlation
It's not going to come as all that much of a shock to most people if there's an association between liking Wicked – the Musical and homosexuality among men, for example. It would be easy to conclude that all the researchers have managed to achieve is letting computers pick up what's already obvious to humans. Easy, but wrong.
Take for example sales of barbecues and hospital cases of sunburn: as one rises, so does the other. We could conclude that barbecues cause sunburn, but it would be more sensible to reason that both factors tend to increase when it's hot. The two have nothing directly to do with each other.
From changed purchasing patterns of seemingly unrelated items, supermarkets are, on occasion, able to infer that a woman is pregnant – sometimes even before she knows herself.
