60. A recent Gallup telephone poll found that 68% of adult Americans favor teaching creationism along with evolution in public schools. The Gallup press release states:

For results based on samples of this size, one can say with 95% confidence that the maximum error attributable to sampling and other random effects is plus or minus 3 percentage points.

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