The Wayback Machine - https://web.archive.org/web/20231127220444/https://www.pewresearch.org/our-methods/u-s-surveys/the-national-public-opinion-reference-survey-npors/

NPORS is an annual survey of U.S. adults conducted by Pew Research Center. Respondents can answer either by paper or online, and they are selected using address-based sampling from the USPS’s computerized delivery sequence file. Respondents are not required to join a survey panel. Pew Research Center uses NPORS to produce benchmark estimates for several topics, such as Americans’ political and religious affiliations