Janice received a telephone call from a research organization asking if she would be willing to answer a few questions about how often she buys certain types of groceries. Janice told the researcher that she was not interested in participating.
Nevertheless, two days later she received another phone call from the same organization asking if she would participate in their survey about buying certain types of groceries. Once again, Janice refused. When the same organization called her for a third time, Janice became irate. She told the researcher "No matter how many times you call me, I will not participate in your survey! Please do not call this number again!"
Which of the following best describes the type of bias Janice (and others liker her) may have introduced into the research organization's survey results?