A study conducted by Health Inc. claims that individuals who consume green tea daily have lower levels of stress compared to those who do not. To support this claim, the researchers noted that 80% of their sample of daily green tea drinkers reported feeling less stressed. However, the researchers also found that 60% of the non-drinkers in the sample were stressed.
Which of the following statements, if true, would most strengthen the argument that drinking green tea reduces stress levels?