The analysis in this report is based mostly on a survey conducted November 12-21, 2014 among a sample of 1,835 adults (921 women and 914 men) 18 years of age or older. The survey was conducted by the GfK Group using KnowledgePanel, its nationally representative online research panel. KnowledgePanel members are recruited through probability sampling methods and include both those with internet access and those without (KnowledgePanel provides internet access for those who do not have it and, if needed, a device to access the internet when they join the panel). A combination of random digit dialing (RDD) and address-based sampling (ABS) methodologies have been used to recruit panel members (in 2009 KnowledgePanel switched its sampling methodology for recruiting panel members from RDD to ABS). The panel includes households with landlines and cellular phones, including those only with cell phones, and those without a phone. Both the RDD and ABS samples were provided by Marketing Systems Group (MSG). KnowledgePanel continually recruits new panel members throughout the year to offset panel attrition as people leave the panel. The survey was conducted in English and Spanish.
All active members of the GfK panel were eligible for inclusion in this study. In all, 3,268 panelists (1,634 women and 1,634 men) were invited to take part in the survey. All sampled members received an initial email to notify them of the survey and provide a link to the survey questionnaire. Additional follow-up reminders were sent to those who had not yet responded as needed.
The final sample of 1,835 adults was weighted using an iterative technique that matches gender and, within gender, age, race/ethnicity, education, region, household income, home ownership status and metropolitan area to parameters from the July 2013 Census Bureau’s Current Population Survey (CPS). In addition, the sample is weighted to match current patterns of internet access from the July 2011 CPS survey. This weight is multiplied by an initial sampling or base weight that corrects for differences in the probability of selection of various segments of GfK’s sample and by a panel weight that adjusts for any biases due to nonresponse and noncoverage at the panel recruitment stage (using all of the parameters described above). Details about the GfK panel-level weights can be found at:
http://www.gfk.com/Documents/GfK-KnowledgePanel-Design-Summary.pdf
Sampling errors and statistical tests of significance take into account the effect of weighting at each of these stages. The margin of sampling error at the 95% confidence level is plus or minus 2.4 percentage points for results based on the full sample (n=1,835). The margin of sampling error is plus or minus 3.4 percentage points for women (n=921) and plus or minus 3.5 percentage points for men (n=914). Sample sizes and sampling errors for other subgroups are available upon request.
In addition to sampling error, one should bear in mind that question wording and practical difficulties in conducting surveys can introduce error or bias into the findings of opinion polls.
Additional Data Source
In addition to the online survey described above, this report is supplemented with public opinion data from an omnibus survey conducted November 20-23, 2014 among a national sample of 1,004 adults, 18 years of age or older, living in the continental United States (501 respondents were interviewed on a landline telephone, and 503 were interviewed on a cell phone, including 291 who had no landline telephone). The survey was conducted by interviewers at Princeton Data Source under the direction of Princeton Survey Research Associates International. A combination of landline and cell phone random digit dial samples were used; both samples were provided by Survey Sampling International. Interviews were conducted in English and Spanish. Respondents in the landline sample were selected by randomly asking for the youngest adult male or female who is now at home. Interviews in the cell sample were conducted with the person who answered the phone, if that person was an adult 18 years of age or older.
The combined landline and cell phone sample are weighted using an iterative technique that matches gender, age, education, race, Hispanic origin and region to parameters from the 2012 Census Bureau’s American Community Survey and population density to parameters from the Decennial Census. The sample also is weighted to match current patterns of telephone status (landline only, cell phone only, or both landline and cell phone), based on extrapolations from the 2013 National Health Interview Survey. The weighting procedure also accounts for the fact that respondents with both landline and cell phones have a greater probability of being included in the combined sample and adjusts for household size among respondents with a landline phone. Sampling errors and statistical tests of significance take into account the effect of weighting.
The margin of sampling error at the 95% confidence level is plus or minus 3.5 percentage points for results based on the full sample (n=1,004). The margin of sampling error is plus or minus 4.9 percentage points for women (n=511) and plus or minus 5.0 percentage points for men (n=493). Sample sizes and sampling errors for other subgroups are available upon request.
