National statistical agencies publicly release information about a nation's population that has been collected under a pledge of confidentiality. A population element which has a unique combination of characteristics and is represented in a sample of microdata where those characteristics appear as categorical variables is at risk of disclosure. An intruder could match the element's unique combination of variables on the microdata to the same combination of variables on some other data base containing identifiers and thus link the element to its microdata record. The percent of unique population elements on a microdata file can be regarded as one component of a measure of disclosure risk. In this paper, two methods of estimating the percent of unique population elements on a sample microdata file using information from that sample are presented and evaluated. A third method of estimation was discussed by Willenborg, Mokken, and Pannekoek (1990) and is reviewed here.