An investigator reviewed the medical records of 200 children seen for care at Boston Medical Center in the past year who were between ages of 8 and 12 years old and identified 40 with asthma. He also identified 40 children of the same age who were free of asthma. Each child and his or her family were interviewed to assess whether there might be an association between exposure to second-hand smoke and asthma. Classify this study as a