Abstract: Methods, systems, and apparatus, including computer programs encoded on a computer storage medium, for determining a number of unique viewers of a content item. In one aspect, a method includes receiving an identifier specifying characteristics of a user associated with a device that was used to present a given content item. A user category is identified for the received identifier based on the characteristics. Each user category of a set of user categories include a respective number of pseudo-users. The received identifier is assigned to a given pseudo-user in the identified user category based on a non-linear distribution of identifiers per actual user in the identified user category. A number of unique users, including the given pseudo-user, that have viewed the given content item is determined based on a total number of pseudo-users that have been assigned an identifier corresponding to presentation of the given content item.