How many epidemiologists does it take to change a lightbulb?

“We’ve found 12,000 switches hidden around the house. Some of them turn this lightbulb on, some of them don’t; some of them only work sometimes; and some of them work sometimes, but twenty years after you flick them. Some of the switches only work, sometimes, twenty years later, if one of the other switches is flicked too (and at the right time). In any case the wiring’s rusty, everything’s completely different in the house next door, and by the way there are lots of people selling spare bulbs who tell lies about houses, switches, and fingers.”

From Ben Goldacre in Bad Science.