Methodology
This SurveyMonkey survey was conducted June 2-8, 2016 online among a national
sample of 3,444 adults aged 18 and over, including 3,048 who say they are
registered to vote. Respondents for this survey were selected from the nearly three
million people who take surveys on the SurveyMonkey platform each day. Data for
this survey has been weighted for age, race, sex, education, region, and voter
registration status using the Census Bureau and Bureau of Labor Statistics’ Current
Population Survey to reflect the demographic composition of the United States.
Because the sample is based on those who initially self-selected for participation
rather 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, coverage error, and measurement error.
To assess the variability in the estimates and account for design effects, we create a
bootstrap confidence interval to produce an error estimate. The bootstrap
confidence interval for this survey is plus or minus 2 percentage points for the full
sample and 2.5 percentage points for registered voters.
To calculate the bootstrap confidence interval we use the weighted data to generate
1000 independent samples 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,
Candidate A could be supported by as low as 44 percent of voters and Candidate B
could have as high as 47 percent of support. Therefore, Candidate A does not have
a "lead."