Is the Nurses’ Health Study an observational study or an experiment? Why?

Observation or experiment? The Nurses’ Health Study has queried a sample of over 100,000 female registered nurses every two years since 1976. Beginning in 1980, the study asked questions about diet, including alcohol consumption. The researchers concluded that “light-to-moderate drinkers had a significantly lower risk of death” than either nondrinkers or heavy drinkers.

(a) Is the Nurses’ Health Study an observational study or an experiment? Why?

(b) What does “significant” mean in a statistical report?

(c) Suggest some lurking variables that might explain why moderate drinkers have lower death rates than nondrinkers. (The study did adjust for these variables.)