Determine whether the study is an observational study or an experiment. Explain.

(a) Researchers conduct a study to determine whether body mass index (BMI) influences mortality. To conduct the study, researchers obtained the BMIs of 3,951,455 people.

(b) Researchers conduct a study to determine whether taking a multivitamin daily affects cognitive health among men as they age. To perform the study, researchers studied 5947 male physicians ages 65 years or older and had one group take a multivitamin daily and had another group take a placebo daily.