ljcox, science does seek the truth, but you worded the following text in a non-scientific way. Agreed
We do not know that smoking causes lung cancer with everyone, we know that people that smoke have an increased rate of lung cancer, for specific cases it is often impossible to prove what caused what, cancer is often fickle in who it strikes. This doesn't mean that smoking can't cause cancer but you have to be very precise in the wording you use.
If smoking caused cancer then why do some 90 year olds die of old age after smoking for the bulk of their life? There is obviously more going on.
Yes, genes have an effect. For example some people can eat a lot of fat and not have a heart attack. But some can't.
Some people are immune to some diseases and some are not.
My point is that statistics show what proportion of the population are susceptable.
In order to be credible, you need large sample sizes.
The research must be repeatable - ie. a different group of researchers must be able to obtain similar results using the same methodology.
Ideally it should be done in different areas & countries.
It is also more credible if similar results can be achieved using different methodologies.