

It matters enormously.
Not for the purpose of establishing whether there is a negative correlation between smoking and life expectancy.
Also that comic is saying that adding a third variable does not always make the model better. Also p-hacking is a thing. It’s the opposite of what you’re trying to establish, which is the existence of a third variable which improves the model. And once again, I will say that if you find two correlated variables confusing, adding a third variable is NOT going to help you.







No, only if you eliminate the risk. Smoking is still negatively correlated with life expectancy in Japan. The correlation exists if you only look at the USA. It exists if you only look at Japan. It exists if you look at the global population.