1.5 Observational studies and experiments. Researchers at The Ohio State University studied the effect of acetaminophen on emotion. The study included a total of 167 volunteers. Approximately half were randomly assigned to take acetaminophen and the other half a placebo. All subjects were shown a variety of images intended to invoke emotions. “Whether they saw things likely to make them feel bad or make them feel good, the acetaminophen group had a less-intense response. It’s not that they didn’t feel sad or happy, just that they felt less (by about 10 to 20%) than their placebo-swallowing peers. It doesn’t turn them into zombies. It doesn’t turn them into robots,” one of the researchers said. Which of the following is true?
(a) This is an observational study, and participants were volunteers.
(b) This is an observational study, and participants were selected at random.
(c) This is an experiment, but participants themselves decided whether to take acetaminophen or the placebo.
(d) This is an experiment, and participants were randomly assigned to treatments.