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METHODOLOGY

The NBC News Online Poll was conducted online by SurveyMonkey October 13-15, 2015 among a national sample of 4,898 adults aged 18 and over. Respondents for this non- probability survey were selected using an algorithm from among the nearly three million people who take surveys on the SurveyMonkey platform each day. Data for this survey have been weighted for age, race, sex, education, and region using the Census

Bureau’s

American Comm unity Survey

to reflect the demographic composition of the United States. Because the sample is based on those who initially self-selected for participation rathe r than a probability sample, no estimates of sampling error can be calculated. All surveys may be subject to multiple sources of error, including, but not limited to sampling error, coverag e error, and measurement error . Polls conducted in one day may include additional error due to the limited time period that panelists have to respond to and complete the survey. To assess the variability in the estimates and account for design effects, we create a bootstrap confidence interval to produce an error estimate, meaning we use the weighted data to generate 5000 independent sample s and calculate the 95% confidence intervals for the weighted average. When analyzing the survey results and their accuracy, this error estimate should be taken into consideration in much the same way that analysis of probability polls takes into account the margin of sampling error. For example, if 47 percent of voters say they support Candidate A and 43 percent of voters support Candidate B, and the error estimate is plus or minus 3.5 percentage points, Candidat e A could be supported by as l ow as 44 percent of voters and Candidate B could have as high as 47 percent of support. Therefor e, Candidate A does not have a "lead." The following table provides the unweighted sample sizes and the error estimate that has been calculated in place of the margin of sampling error for a variable that is expected to have close to an even split in most groupings (such as gender):