Why "cancer clusters" are so hard to confirm

This excerpt from the new book, Toms River by Dan Fagin, has me instantly intrigued. The book is about one of the rare places where scientists were able to prove that not only was there a cluster of cancer cases, but that those cases could be linked to a cause. The excerpt explains why this is such a rare thing. Turns out, just because it looks like a town has more cancers than it should, doesn't mean that's always what's going on.