The Associated Press-GfK poll on tap water safety was conducted by GfK Public Affairs and Corporate Communications on Feb. 11-15. It is based on online interviews of 1,033 adults who are members of GfK's nationally representative KnowledgePanel.
The original sample was drawn from respondents recruited by phone or mail survey methods. GfK provides Internet access to participants who don't already have it. With a probability basis and coverage of people who otherwise couldn't access the Internet, online surveys using KnowledgePanel are nationally representative.
Interviews were conducted in both English and Spanish.
As is done routinely in surveys, results were weighted, or adjusted, to ensure that responses accurately reflect the population's makeup by factors such as age, sex, race, education and phone use.
No more than 1 time in 20 should chance variations in the sample cause the results to vary by more than plus or minus 3.4 percentage points from the answers that would be obtained if all adults in the U.S. were polled.
There are other sources of potential error in polls, including the wording and order of questions.
The poll had a cooperation rate of 46 percent.
The questions and results are available at: http://www.ap-gfkpoll.com