From December 2007 through March 2012, researchers at the University of Pittsburgh studied a new medication to treat osteoporosis in frail elderly women. The researchers administered a daily dose of the medication or a placebo by injection along with calcium and vitamin D supplements. Then the researchers measured the hip and spine bone density of each patient at 12 and 24 months. Results were published in the Journal of the American Medical Association (JAMA) in April 2015. This is an example of which type of study? observational study experiment