The analysis in this report is based on an online survey conducted September 12-18, 2014 among a sample of 1,066 adult internet users, 18 years of age or older. The survey was conducted by the GfK Group using KnowledgePanel, its nationally representative online research panel. To qualify for the main survey, a panel member must have been at least 18 years of age and meet one of the following two criteria:
- Use the internet or email at least occasionally; or
- Access the internet on a cell phone, tablet or other mobile handheld device at least occasionally
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 comprises 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. Respondents were selected randomly from eligible adult household members of the panel. All sampled members received an initial email September 12-13, 2014 to notify them of the survey and included a link to the survey questionnaire. One follow-up reminder was sent three days later to those who had not yet responded.
The final sample for this survey was weighted using an iterative technique that matches gender, age, education, race, Hispanic origin, household income, metropolitan area or not, and region 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 base or sampling weight that corrects for differences in the probability of selection of various segments of the 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 mentioned above as well home ownership status).
Sampling errors and statistical tests of significance take into account the effect of weighting at each of these stages. Sampling error for the total sample of 1,066 respondents is plus or minus 3.2 percentage points at the 95% level of confidence. 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.