Which of These Things Is More Likely Than the Others

Which of these things is more likely than the others to undermine the validity of survey research?

A) Researchers purposely avoid discussion of controversial issues with people who are part of a prospective random sample.
B) People who are part of a prospective random sample screen their phone calls.
C) People who are part of a prospective random sample are contacted as close to Election Day as possible.
D) Researchers ask people who are part of a prospective random sample too many open-ended questions.
E) Researchers have to try several times to get through to people who are part of a prospective random sample.