Errors in surveys. A Gallup Poll found that 30% of adult Americans claim to have postponed medical treatment due to cost. The Gallup press release says:

Results are based on telephone interviews with 1,014 national adults, aged 18 and older, conducted Nov. 11–14, 2007. For results based on the total sample of national adults, one can say with 95% confidence that the maximum margin of error is ±3 percentage points.

The release also points out that this margin of error is due only to sampling error. Give one example of a source of error in the poll result that is not included in this margin of error.