Summary: Math 140
Professor Bernardo Ábrego
8.1 Estimating a Proportion
A recent Phi Delta Kappa/Gallup poll reported that a record 51%
of the American public assigns a grade of A or B to the public
schools in their community and that this survey had a margin of
error of 3%. Source: 2001, www.gallup.com/poll/releases/pr010823.asp.
These results are based on telephone interviews with a
randomly selected national sample of 1108 adults, 18 years and
older, conducted May 23June 6, 2001.
For results based on this sample, one can say with 95 percent
confidence that the maximum error attributable to sampling and
other random effects is plus or minus 3 percentage points. In
addition to sampling error, question wording and practical
difficulties in conducting surveys can introduce error or bias into
the findings of public opinion polls.