Bias in calculation of attributable fractions using relative risks from nonsmokers only.

Studies of weight and mortality sometimes state that the mortality relative risks for obesity from nonsmokers are valid estimates of the relative risks for obesity in both smokers and nonsmokers. Extending this idea, several influential articles have used relative risks for obesity from nonsmokers and attributable fraction methods for unadjusted risks to estimate attributable fractions of deaths in the entire population (smokers and nonsmokers combined). However, stratification by smoking is a form of adjustment for confounding. Simplified examples show that the use of relative risks from only 1 stratum to estimate attributable fractions, without incorporating data on the stratification variable, gives incorrect results for the entire population. Even if the mortality relative risks for obesity from nonsmokers are indeed valid in both smokers and nonsmokers, these relative risks nonetheless need to be treated as adjusted relative risks for the purpose of calculating attributable fractions for the whole sample.