Our intuitive acuity about statistical correlations, like the acuity of the human eye, performs best at the margins. When rare events are superposed against rare events, the association between them can be striking. Pott, for instance, had discovered the link between scrotal cancer and chimney sweeping because chimney sweeping (the profession) and scrotal cancer (the disease) were both uncommon enough that the juxtaposition of the two stood out starkly like a lunar eclipse—two unusual occurrences in precise overlap.
