For further information, please refer to the US Census Bureau's Annual Services release, online at http://www.census.gov/services/.
Source: US. Bureau of the Census
Release: Annual Services Survey
US. Bureau of the Census, Defined Benefit Pension Plans for Freestanding Ambulatory Surgical and Emergency Centers, All Establishments, Employer Firms (DISCONTINUED SERIES) [EXPDBPEF621493ALLEST], retrieved from FRED, Federal Reserve Bank of St. Louis https://research.stlouisfed.org/fred2/series/EXPDBPEF621493ALLEST/, July 28, 2015.