Comment on the ethics of the following situations:
a. A researcher plans to use invisible ink to code his direct mail questionnaires to identify those respondents who return the questionnaire.
b. A telephone interviewer calls at 10:00 p.m. on a Sunday and asks to conduct an interview.
c. A manufacturer purchases 100,000 e-mail addresses from a national e-mail distribution house and plans to e-mail out a short sales promotion under the heading “We Want to Know Your Opinions.”