balance
they need to balance the identifiers with anonymity, each identifier could be unique to that field value only re-identifiable by secured DB lookup but then the value of the data set could be worthless if the original identifiable information is missing or obfuscated to the point of being meaningless. Yes the resolution of the identifiers could be reduced but doesn't help when looking for things that are so rare that casual observation of its occurrence in a data set identifies the person it references.
Its hard.
