There's an old post about the Anime Death Note and the "bits of entropy" in relation to anonimity. It boils down to enough true/false questions about a person is enough information to uniquely identify them.
> The researchers generalized their Netflix work to find isomorphisms between arbitrary graphs (such as social networks stripped of any and all data except for the graph structure), for example Flickr and Twitter , and give many examples of public datasets that could be de-anonymized—such as your Amazon purchases (Calandrino et al 2011 ; blog).
