Back Pain Treatment In a study designed to test the effectiveness of paracetamol (also known as acetaminophen) as a treatment for lower back pain, 1643 patients were randomly assigned to one of three groups: (1) the 547 subjects in the placebo group were given pills containing no medication; (2) 550 subjects were in a group given pills with paracetamol taken at regular intervals; (3) 546 subjects were in a group given pills with paracetamol to be taken when needed for pain relief. (See “Efficacy of Paracetamol for Acute Low-Back Pain,” by Williams et al., Lancet.) Is this study an experiment or an observational study? Explain.