Reanalysis of the Mayo Lung Project data: the impact of confounding and effect modification

Objectives-To examine whether age at entry, history of cigarette smoking, e xposure to non-tobacco lung carcinogens, or previous pulmonary illnesses we re confounders or effect modifiers of the relation between screening and lu ng cancer mortality in the Mayo Lung Project. Setting-The Mayo Lung Project was a randomised, controlled, clinical trial conducted between 1971 and 1986 in 9211 male smokers over the age of 45 in Minnesota (USA). The group screened received chest x ray examination and sp utum cytology every four months for six years. The unscreened group were re commended to obtain usual care (annual chest x ray examination and sputum c ytology). After follow up, lung cancer mortality was similar in both groups . Methods-Proportional hazard models were used to analyse data. A variable wa s considered a confounder if its inclusion in a model changed the rate rati o for screening by more than 15%; a variable was considered an effect modif ier if its stratum-specific rate ratio for screening differed by a factor o f two. Results-None of the four aforementioned variables changed the rate ratio as sociated with screening (1.07) by more than 2%. The effect of screening may have differed by years smoked (rate ratio for smoking fewer than 30 years 2.4; rate ratio for smoking 30 or more years 1.0), though we suspect that t his result occurred by chance. Conclusion-Adjustment for or stratification by four established lung cancer risk factors did not alter the original findings of the Mayo Lung Project.