Methodology › 2018 Methodology

2018 Methodology

The 2018 American Values Atlas (AVA) is a project of PRRI. Results for all demographic, religious affiliation, and political affiliation questions were based on 54,357 bilingual telephone interviews (including 32,569 cell phone interviews) conducted between January 3, 2018 and December 30, 2018 by professional interviewers under the direction of SSRS. Results for questions on specific issues (e.g., LGBT issues) are based on a subset of 40,292 telephone interviews (including 24,149 cell phone interviews) conducted between March 14, 2018 and December 16, 2018. Results for questions on same-sex marriage are based on a subset of 4,028 telephone interviews (including 2,413 cell phone interviews) conducted between March 14, 2018 and March 25, 2018 and between June 27, 2018 and July 8, 2018. The AVA was made possible by a generous grant from an anonymous donor with additional support from The Nathan Cummings Foundation, The Evelyn and Walter Haas, Jr. Fund, The Gill Foundation, and United Universalist Veatch Program at Shelter Rock.

Throughout 2018, at least 1,000 interviews were completed each week, with about 600 interviews conducted among respondents on their cell phones. Each week, interviewing occurred over a five- or six-day period, starting Tuesdays or Wednesdays and going through Sunday or Monday. The selection of respondents within households was accomplished by randomly requesting to speak with the youngest adult male or female currently living in the household.

Data collection was based on stratified, single-stage, random-digit-dialing (RDD) of landline telephone households and randomly generated cell phone numbers. The sample was designed to represent the total U.S. adult population from all 50 states, including Hawaii and Alaska. The landline and cell phone samples were provided by Marketing Systems Group.

The weighting was accomplished in two separate stages. The first stage of weighting corrects for different probabilities of selection associated with the number of adults in each household and each respondent's telephone usage patterns. In the second stage, sample demographics were balanced to match target population parameters for gender, age, education, race and Hispanic ethnicity, region (U.S. Census definitions), population density, and telephone usage. The population density parameter was derived from 2010 Census data. The telephone usage parameter came from an analysis of the January-June 2017 National Health Interview Survey. All other weighting parameters were derived from an analysis of the U.S. Census Bureau's March 2017 Current Population Survey.

The sample weighting was accomplished using iterative proportional fitting (IFP), a process that simultaneously balances the distributions of all variables. Weights are trimmed so that they do not exceed 4.0 or fall below 0.25 to prevent individual interviews from having too much influence on the final results. The use of these weights in statistical analysis ensures that the demographic characteristics of the sample closely approximate the demographic characteristics of the target populations.

The table below shows the sample sizes for all 50 states and 30 metro areas for both sample and issue subsample. The margin of error for the total sample is +/- 0.4 percentage points at the 95% level of confidence. The margin of error for issue subsample is +/- 0.5 percentage points at the 95% level of confidence. The margin of error for the same-sex marriage question subsample is +/-1.5 percentage points at the 95% level of confidence. The design effect for the total sample, the issue subsample, and the same-sex marriage question subsample is 1.4. In addition to sampling error, surveys may also be subject to error or bias due to question wording, context, and order effects.

Sample Sizes for States and Metro Areas

State Total Sample Issue Subsample
United States 54,357 40,292
Alabama 839 633
Alaska 277 224
Arizona 1,237 931
Arkansas 547 396
California 5,578 4,200
Colorado 880 658
Connecticut 543 397
Delaware 183 143
Florida 3,455 2,580
Georgia 1,633 1,228
Hawaii 226 171
Idaho 359 261
Illinois 1,723 1,258
Indiana 1,237 904
Iowa 723 526
Kansas 547 394
Kentucky 771 575
Louisiana 692 492
Maine 303 224
Maryland 949 706
Massachusetts 1,049 789
Michigan 1,711 1,292
Minnesota 1,070 763
Mississippi 395 296
Missouri 1,104 784
State Total Sample Issue Subsample
Montana 300 226
Nebraska 435 294
Nevada 472 338
New Hampshire 270 204
New Jersey 1,299 954
New Mexico 406 292
New York 3,410 2,553
North Carolina 1,730 1,274
North Dakota 187 142
Ohio 2,065 1,541
Oklahoma 652 476
Oregon 1,006 754
Pennsylvania 2,309 1,744
Rhode Island 164 121
South Carolina 822 619
South Dakota 184 142
Tennessee 1,192 867
Texas 3,853 2,780
Utah 646 498
Vermont 179 128
Virginia 1,503 1,127
Washington 1,433 1,044
West Virginia 401 301
Wisconsin 1,079 802
Wyoming 136 104
Metro Area Total Sample Issue Subsample
United States 54,357 40,292
Albany 206 140
Albuquerque 184 140
Allentown 168 135
Anchorage 143 113
Atlanta 895 690
Austin 320 232
Baltimore 485 349
Birmingham 207 152
Boston 729 552
Buffalo 235 182
Charleston 140 105
Charlotte 345 261
Chicago 1,182 874
Cincinnati 374 271
Cleveland 328 246
Columbus 356 255
Dallas 931 692
Dayton 157 112
Denver 404 308
Detroit 700 526
El Paso 141 108
Fresno 176 131
Grand Rapids 177 134
Greensboro 143 103
Greenville 142 103
Hartford 181 142
Houston 849 603
Indianapolis 361 277
Jacksonville 291 216
Kansas City 346 236
Knoxville 176 134
Las Vegas 301 211
Los Angeles 1,643 1,224
Louisville 253 198
Memphis 199 140
Miami 820 595
Milwaukee 300 226
Minneapolis-St. Paul 663 467
Nashville 286 206
New Orleans 190 138
New York City 3,085 2,295
Oklahoma City 205 155
Omaha 184 134
Orlando 357 278
Philadelphia 972 723
Phoenix 792 588
Pittsburgh 438 315
Portland 473 358
Providence 245 192
Raleigh 207 143
Richmond 206 153
Riverside 667 485
Rochester 212 167
Sacramento 347 271
Salt Lake City 260 197
San Antonio 321 223
San Diego 509 390
San Francisco 639 476
San Jose 249 187
Seattle 635 465
Spokane 148 111
St. Louis 484 362
Tampa-St. Petersburg 487 373
Tucson 184 143
Tulsa 168 125
Virginia Beach 317 241
Washington, DC 1,043 776
Worcester 148 107