Medicare's common denominator: the covered population.

This report describes Medicare eligibility requirements: the processes to establish Medicare entitlement; types of coverage; the composition of the enrolled population; and outlines some differences in measurement techniques used in a decennial census in contrast to Medicaid enrollment. Current Medicare enrollment figures do not represent a complete count of any segment of the total United States population. Some persons age 65 and over are not eligible for Medicare; others are eligible but not entitled. However, the Medicare enrollment figures are frequently used as surrogate counts of the aged population because they provide excellent sources of detailed demographic and geographic information for a large proportion of those age 65 and over. The data are produced semi-annually, based on daily updates.