"Ann Landers" is famous for giving advice. Once, in response to a letter from an engaged couple, she asked her readers to write and respond to the question "If you could do it over again, would you have children?" She received over 10,000 responses, 70% of whom said they would not have had children if they could do it over again. These results cannot be generalized to the entire population of parents due to voluntary response, making this a _____ study.

(Answer is NOT voluntary study)