According to the study results, men who consumed antioxidant supplements for <5 years and men who consumed antioxidant supplements for ≠¥5 years have stroke relative risks of 0.95 (p = 0.45) and 0.75 (p < 0.01), respectively, when compared to the reference group. The results of the study were adjusted to account for baseline differences related to healthy behaviors and overall health. Which of the following factors most likely explains why the relative risk of stroke is lower with longer antioxidant use?
1) Increased overall health
2) Improved healthy behaviors
3) Longer duration of antioxidant use
4) Reference group characteristics