A large geographical area in the rural town of Mandeville

in Jamaica tended to have great outbreaks of gastrointestinal illnesses among

its population during the hurricane season. Researchers wanted to find out if the

number of cases of illnesses had anything to do with where the inhabitants collected

their drinking water. Many rural inhabitants used the river sources for their

water supply throughout the area. Researchers decided to have a group of the

population collect water only from the top or head of the river, another group only

from the middle, a third group only from the bottom of the river, and a fourth

group used only bottled water. They then assessed the number of cases of illness

that resulted from those drinking from each section or group. The researchers’

activities BEST relate to which level of research?