The National Health and Aging Trends Study (NHATS) was designed to follow successive nationally representative cohorts of persons ages 65 and older and enrolled in Medicare. Round 1 of the study was fielded in 2011; in Round 5 (2015) the sample was replenished. The design includes oversamples of individuals at older ages and those whose race is listed as Black on the CMS enrollment file.
More information on the 2011 NHATS sample design, including detailed information on the sample size, response rates, and survey design can be found in the document, "National Health and Aging Trends Study Round 1 Sample Design and Selection. NHATS Technical Paper #1."
Details on the Round 5 sample design and selection for can be found in "National Health and Aging Trends Study Round 5 Sample Design and Selection. NHATS Technical Paper #16."