Surveys were mailed to every 20th person listed in local telephone directory. Each person was asked to report age, sex, drinking habits, and breast cancer presence during the preceding week. Nearly 40% of the surveys were completed and returned. About 10% of women reported the presence of breast cancer overall. 15% of heavy drinkers reported breast cancer and 8% of teetotalers reported breast cancer.
What research design was likely used to conduct this study?