Santo Fortunato

Two people are talking at once, and by the loudness of their voices you can't tell who is answering whom. That's how climatologists have for decades searched for a link between warm water off the coast of Peru and droughts in Australia: they compared how synchronously ocean temperature and rainfall change in different parts of the planet.

A social network is like a city: there are giant neighborhoods and tiny alleys. When an algorithm looks for tight-knit groups, it often picks out a couple of huge clusters and many small fragments. But the client (say, a municipality) needs districts of roughly equal size to distribute clinics or schools evenly.