Two tea shops stand in the same market. One earns twice as much as the other

Why

The customer has spotted a real difference and reached for the most natural explanation, and it may well be part of the truth — better tea does bring people back. But earnings are the end of a long chain, and taste is only one link in it. The busier shop may sit right at the bus stand while the other is round a corner; it may open at five in the morning; it may have been there thirty years; it may give credit to regulars, or simply have more benches. Any of these doubles takings with identical tea. Test: the closest thing to a fair comparison is to serve both teas to the same people without telling them which is which. Notice the shape of this: whenever one number is bigger than another, the interesting question is what ELSE differs between them. It cannot be concluded — one difference in outcome does not identify which of many differences caused it.