Methods and apparatus to assign demographic information to panelists are disclosed. Example disclosed apparatus include a decoy generator to generate decoy database proprietor identifiers to protect privacies of members of panelist households corresponding to panelist database proprietor identifiers, the decoy database proprietor identifiers to be determined by the decoy generator based on a probability density function so that a quantity of the decoy database proprietor identifiers is less than would be used if the decoy database proprietor identifiers were selected by a random selection process. The example apparatus also includes a query handler to query, via a network, a server at a database proprietor using the panelist database proprietor identifiers and the decoy database proprietor identifiers, to obtain demographic information. The example method also includes a panelist comparator to assign the panelist database proprietor identifiers to panelists in the panelist households based on the demographic information obtained from the server at the database proprietor.