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National Public Opinion Reference Survey, 2021




Smith, G. A. (2022, July 6). National Public Opinion Reference Survey, 2021.


The National Public Opinion Reference Survey (NPORS) is an annual survey of U.S. adults conducted by the Pew Research Center. Respondents can answer either by paper or online, and they are selected using address-based sampling. The Pew Research Center uses NPORS to produce benchmark estimates for several topics, such as Americans' political and religious affiliations.

NPORS was conducted for the Pew Research Center by Ipsos. Participants were first mailed an invitation to complete an online survey. A paper survey was later mailed to those who did not respond. In total, 1,550 respondents completed the survey online and 2,387 respondents completed the paper survey. The survey was administered in English and Spanish.

The ARDA has added five additional variables to the original data set to enhance the users' experience on our site.

Data File

Cases: 3937
Variables: 62
Weight Variable: WEIGHT

The survey was weighted to support reliable inference from the sample to the target population of U.S. adults. The weight was created using a multistep process that includes a base weight adjusting for differential probabilities of selection and a raking calibration that aligns the survey with the population benchmarks. The process starts with the base weight, which accounted for the probability of selection of the address from the U.S. Postal Service Computerized Delivery Sequence File frame, as well as the number of adults living in the household.

Then the base weights are calibrated to population benchmarks using raking, or iterative proportional fitting. The raking dimensions and the source for the population parameter estimates are reported in the table below. All raking targets are based on the non-institutionalized U.S. adult population (ages 18 and older). These weights are trimmed at about the 1st and 99th percentiles to reduce the loss in precision stemming from variance in the weights.

Data Collection

May 29 - Aug. 25, 2021

Original Survey (Instrument)

2021 National Public Opinion Reference Survey

Funded By

Pew Research Center

Collection Procedures

Mailing protocol
Ipsos sent initial mailings in a 9-by-12-inch envelope via First-Class mail to the 13,500 sampled households. These packets included two $1 bills and a letter asking a member of the household to complete an online survey using the website and password provided. If two or more adults were in the household, the letter asked the adult with the next birthday to complete the survey. Sampled households were later sent a reminder postcard and then a reminder letter via first class mail.

Ipsos sent nonresponding households with a deliverable address a 9-by-12-inch Priority mail envelope. The Priority envelope contained a letter, $5 bill, a paper version of the survey and a postage-paid return envelope. The paper survey was one 11-by-17-inch page folded booklet-style.

The within-household selection instructions were identical to those used in the earlier online survey request. These households were later sent a reminder postcard. Households in Hispanic strata received all materials in English and Spanish. All other households received materials in English only. Those who completed the survey online or returned the
completed paper survey were sent a $10 post-incentive.

Questionnaire development and testing
Pew Research Center developed the questionnaire in consultation with Ipsos. The online questionnaire was tested on both desktop and mobile devices. The test data was analyzed to ensure the logic and randomizations were working as intended before the survey was launched.

The online questionnaire contains more questions than the paper questionnaire because a subset of respondents to the online survey were eventually asked if they were willing to join the American Trends Panel. More information is needed about adults who join the panel. The NPORS sample, however, includes all adults completing the online or paper questionnaire regardless of whether they agreed to join the panel. The NPORS dataset contains only the questions measured both on paper and online.

Sampling Procedures

The sample was drawn from the U.S. Postal Service Computerized Delivery Sequence File (DSF) and was provided by MSG (Marketing Systems Group). Occupied residential addresses (including 'drop points') in all U.S. states (including Alaska and Hawaii) and the District of Columbia had a nonzero chance of selection. The draw was a national, stratified random sample, with differential probabilities of selection across the mutually exclusive strata. Ipsos designed the sample plan with the goal of obtaining in the recruited panel the distribution of age-by-race groups below:
- Hispanic adult ages 18-24 2.0%
- Hispanic adult ages 25+ 12.0%
- Black adult ages 18-24 1.5%
- Black adult ages 25+ 9.5%
- Other 18-24 (non-Hispanic, non-Black) 4.0%
- High school or less (any age, ethnicity) 20.0%
- All other adults ages 25+ 51.0%

Weighting and survey design features that depart from simple random sampling tend to result in an increase in the variance of survey estimates. This increase, known as the design effect, or 'deff,' should be incorporated into the margin of error, standard errors and tests of statistical significance. The overall design effect for a survey is commonly approximated as 1 plus the squared coefficient of variation of the weights. For this survey, the margin of error (half-width of the 95% confidence interval) incorporating the design effect for full sample estimates at 50% is plus or minus 2.1 percentage points. Estimates based on subgroups will have larger margins of error. It is important to remember that random sampling error is only one possible source of error in a survey estimate. Other sources, such as question wording and reporting inaccuracy, may contribute additional error.

Principal Investigators

Gregory A. Smith

Related Publications

Pew Research Center. (2021). "About Three-in-Ten U.S. Adults Are Now Religiously Unaffiliated."

Pew Research Center. (2021). "How Pew Research Center Uses Its National Public Opinion Reference Survey (NPORS)."

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