1. A method to determine an unknown attribute of a user, the method comprising: producing, by executing first instructions on a processor, a reduced attribute vector based on an unclassified attribute vector and a projection matrix, the unclassified attribute vector including a known attribute value of the user and an unknown attribute value of the user, the projection matrix generated based on a correlation between a plurality of attribute vectors for a first plurality of users and a plurality of attribute vectors for a second plurality of users on a panel, and the reduced attribute vector having fewer attributes than the unclassified attribute vector; producing, by executing second instructions on the processor, a plurality of reduced panel attribute vectors based on a plurality of classified panel attribute vectors of the second plurality of users from the panel and the projection matrix, wherein the reduced panel attribute vectors have fewer attributes than the classified panel attribute vectors, and the user is not one of the second plurality of users from the panel; and determining, by executing third instructions on the processor, the unknown attribute of the user based on the reduced attribute vector, the plurality of reduced panel attribute vectors, and known attributes of the second plurality of the users of the panel, wherein the known attributes correspond to different reduced panel attribute vectors.