"Drinking water rich in minerals, or hard water, may play a role in reducing heart disease, Finnish researchers say."43 For one of these study designs, it would be appropriate to summarize results with a correlation; which is it? For the other designs, tell why correlation would not be appropriate.
a. Take a sample of individuals from around the country; record the hardness (parts per million of calcium carbonate) of water where they live, and whether they die of a heart attack.
b. Take a sample of communities from around the country; record the hardness (parts per million of calcium carbonate) of water in each community, and the rate of death by heart attack.
c. Take a sample of communities from around the country; record whether the water in each community is hard or soft, and record the rate of death by heart attack in each community.
d. Take a sample of individuals from around the country; record whether the water where they live is hard or soft, and whether they die of a heart attack.