Many medical experts have suggested that excessive coffee drinking is harmful to health. In order to verify the impact of excessive coffee drinking on the heart, researchers selected two groups of subjects. The first group of subjects drank five or more cups of coffee a day, and the second group of subjects never drank coffee. After a period of time, it was found that the proportion of heart disease in the first group of subjects was significantly higher than that in the second group. 
Some of the subjects in the first group drank 10 cups of coffee a day
 The researchers concluded that drinking too much coffee increased the risk of heart disease.