An official website of the United States government
The .gov means it’s official.
Federal government websites often end in .gov or .mil. Before
sharing sensitive information, make sure you’re on a federal
government site.
The site is secure.
The https:// ensures that you are connecting to the
official website and that any information you provide is encrypted
and transmitted securely.
Couper MP, Singer E, Conrad FG, Groves RM.Couper MP, et al. Among authors: conrad fg.J Off Stat. 2008;24(2):255-275.J Off Stat. 2008.PMID: 21603156Free PMC article.
Couper MP, Singer E, Conrad FG, Groves RM.Couper MP, et al. Among authors: conrad fg.J Off Stat. 2010 Jun;26(2):287-300.J Off Stat. 2010.PMID: 21765576Free PMC article.
Peytchev A, Conrad FG, Couper MP, Tourangeau R.Peytchev A, et al. Among authors: conrad fg.J Off Stat. 2010;26(4):633-650.J Off Stat. 2010.PMID: 23411499Free PMC article.