Each spring, high school students, college interns, and retirees count and weigh tiny glass eels in streams that feed into the Hudson River. They report their observations to the New York State Department of Environmental Conservation Hudson River Estuary Program and the National Estuarine Research Reserve System, who depend on volunteers to help build a database of information about glass eels. The observations that these volunteers make would best be described as . a hypothesis-driven scientific investigation the scientific method a controlled experiment observational studies