Town A and Town B are both located in Massachusetts. Both towns have approximately 100,000 residents, and their own water supply. Fluoride is added to the water supply of Town A; nothing is added to the water supply of Town B. The decision to add fluoride to Town A's water, and not Town B's, was made using a random number table. Both towns are monitored and compared with regards to the occurrence of new cases of dental caries (cavities) over a 5 year period. What type of study is this