The analysis in this report is based on a survey conducted June 17-27, 2016, among a sample of 1,055 adult internet users 18 years of age or older. The survey was conducted in English and Spanish 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 cellphones and those without phones. KnowledgePanel continually recruits new panel members throughout the year to offset panel attrition as people leave the panel.
To qualify for the survey, a panel member must have been 18 years of age or older and a current internet user. In all, 1,804 panelists were invited to take part in the survey. Of 1,088 cases completing the main survey, 1,055 were determined to be valid cases to be included in the final analyses. The remaining 33 cases were excluded due to refusing to answer more than one-quarter of the substantive survey questions or completing outside of three standard deviations of the duration time. 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 1,055 adults was weighted using an iterative technique that matches gender and, within gender, age, race/ethnicity, education, region, household income, household internet access and primary language to parameters from the March 2015 Census Bureau’s Current Population Survey (CPS). 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:
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 3.2 percentage points for results based on the full sample (n=1,055). The following table shows the unweighted sample sizes and the error attributable to sampling that would be expected at the 95% level of confidence for different groups in the survey:
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.
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