The New England Journal of Medicine reported on a study of fish oil consumption in pregnant mothers and the subsequent development of asthma in their children. Read the excerpts from the abstract and answer the questions that follow: (Bisgaard et al., “Fish oil–derived fatty acids in pregnancy and wheeze and asthma in offspring,” New England Journal of Medicine, vol. 375 [December 2016]: 2530–2539, doi:10.1056/NEJMoa1503734 Methods: We randomly assigned 736 pregnant women at 24 weeks of gestation to receive fish oil or a placebo (olive oil) daily. Neither the investigators nor the participants were aware of group assignments during follow-up for the first 3 years of the children’s lives, after which there was a 2-year follow-up period during which only the investigators were unaware of group assignments. Results: A total of 695 children were included in the trial, and 95.5% completed the 3-year, double-blind follow-up period. The risk of persistent wheeze or asthma in the treatment group was 16.9%, versus 23.7% in the control group, corresponding to a relative reduction of 30.7%. 

a. Was this a controlled experiment or an observational study? Explain how you know.

b. Assuming the study was properly conducted, can we conclude that the lower rate of asthma was caused by the mother’s consumption of fish oil?