The link between smoking and cancer, for example, was seen in many observational studies, but forcing subjects to smoke for years to prove it would be untenable.

• 拿吸烟和肺癌之间关系的研究来说，许多观察性研究都发现了这一点，但要迫使研究对象吸烟多年来证明这一点恐怕是不可能的。