Often an adversary can gain some partial information about the users of a system, such as the fact that they have high-bandwidth connections or all live in California. Preventing an adversary from obtaining any such information may be impossible. Instead of asking ``is the system anonymous,'' the question shifts to ``is it anonymous enough?''
We might say that a system is partially anonymous if an adversary can only narrow down a search for a user to one of a ``set of suspects.'' If the set is large enough, then it is impractical for an adversary to act as if any single suspect were guilty. On the other hand, when the set of suspects is small, mere suspicion may cause an adversary to take action against all of them.
Independently, Syverson has developed a logic for talking about the adversary's view of a set of suspects [41].