If the "normal" rate of lung cancer is X, the observed rate in nonsmokers who get secondhand smoke is X+Y, and the observed rate in smokers is X+Y+Z, if you compare nonsmokers and smokers it looks like smoking increases your rate by Z when it's actually Y+Z.
alt Hacker News
Yes, exactly my point.
If the "normal" rate of lung cancer is X, the observed rate in nonsmokers who get secondhand smoke is X+Y, and the observed rate in smokers is X+Y+Z, if you compare nonsmokers and smokers it looks like smoking increases your rate by Z when it's actually Y+Z.