case-control studies

Let’s say we’re interested in the effect of smoking on low infant birthweight, where “low infant birthweight” is an indicator based on birthweight falling below 2500 grams. In an experimental study, we would randomly assign expectant mothers to either smoke or not smoke, record their infant’s birthweight, and then compare the proportion of low birthweights between the two groups. Obviously that would be unethical and no review board would approve such a study. We also probably wouldn’t get many participants.