In a study designed to test the effectiveness of paracetamol (or 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, doi:10.1016>S0140-6736(14)60805-9.) Is this study an experiment or an observational study? Explain.