YOU THINK I WILL AGREE THAT EASILY!
Basically, all I am saying is this. You can use sample data to make statistical inferences about the original population from which you sampled the data, but you cannot make such inferences about a different population.
In your example of smoking and lung cancer, you could say that someone who smokes likely has lung cancer. But if you consider only those individuals whose astrological sign is Aries or only those whose blood type is A then your previous observations alone dont give you any reason to say how likely it is that one of those individuals who smokes also has lung cancer. To do that you also need judgement.