this appears to be a quite difficult question. As we know, if 20% of the smokers developed lung cancer and 10% of the non-smokers developed lung cancer, then we say that cigarette smoking is associated with a 2-time (20% divided by 10%) relative risk of developing lung cancer. But if 0% of the non-smokers developed lung cancer, what would the relative risk be? And how would you describe the risk of smoking in this situation for general readers? Thank you!    


Well, you can't give a relative risk if one group has zero risk.

As for describing, I'd recommend that you don't describe the risk of smoking, but rather the advantages of not smoking:

If you don't smoke, you will never die of lung cancer. Period.

However, it may just be that your sample of nonsmokers is somewhat unrepresentative. I'd recommend that you find a few more data points.


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