6.7 Wine and heart health. To explore the effects of red wine on heart health, you recruit 100 volunteers. Half are to drink one glass of red wine a day with dinner for a month. The other half are to abstain from any alcohol for a month. The diets of all volunteers are otherwise the same. Women and men may respond differently to wine. Forty of the volunteers are women and 60 are men, so the researchers separately randomly assign half the women to the wine group and half the men to the wine group. The remaining volunteers are assigned to the no alcohol group. This is an example of