Heterogeneity and other problems in a pooled analysis of snus use and mortality

A recent analysis of Swedish snus use and mortality combined eight Swedish datasets and found that exclusive Swedish male snus users have statistically significant increased mortality from all causes, cardiovascular diseases and other causes. These findings, from the Swedish Collaboration on Health Effects of Snus Use, are in sharp contrast with previous pooled results from the same group. The discrepant results may be indicative of unresolved statistical problems that haven’t been addressed by the collaboration authors in any of their studies. The most important problem is unresolved heterogeneity among the eight cohorts, which we describe in detail, and we show how the use of the random effects method by the authors was not sufficient. We explain why the tables in the article are uninformative, and we demonstrate why the exclusion of smokers in the analysis was not validated and eliminated important information. Finally, we strongly recommend some straightforward and easily implemented corrective measures.