4.11 Not in the margin of error. According to a June 2015 Gallup Poll, more American adults ages 18 to 29 report being single and never married—the percentage has risen from 52% in 2004 to 64% in 2014. For American adults aged 30 to 39, this same percentage has risen modestly from 15% in 2004 to 19% in 2014. The Gallup press release says:

For results based on the total sample of Americans ages 18 to 29 or 30 to 39 in any given year, the maximum margin of sampling error is ±3 percentage points at the 95% confidence level.

Give one example of a source of error in the poll result that is not included in this margin of error.