O Lung cancer mortality rates among heavy smokers.
Explanation:
This question refers to Bradford Hill's criteria for causation. These are nine principles that Bradford Hill developed in order to allow researchers in epidemiology establish evidence of a causal relationship between a presumed cause and an observed effect. This criteria has been widely used in research. The criterion of strength, or effect size, indicates that a small association does not mean that there is not a causal effect, though the larger the association, the more likely that it is causal. As the association between lung cancer mortality rates and heavy smoking is quite high, this is likely to be a causal relationship.
