The Department of Natural Resources (DNR) is planning a study to determine whether breakwaters help decrease erosion at a large lake. They divide the shoreline into 100-foot plots, installing some with breakwaters and some without. However, the east and west shorelines of this lake receive very different wave patterns due to the wind. The DNR suspects this will affect the responses to the breakwaters. Because of this, the DNR plans to treat each shoreline as a different group. This study is an example of