jnci.oxfordjournals.org JNCI | Correspondence 211 Re: False-Positive Results in Cancer Epidemiology: A Plea for Epistemological Modesty

The commentary by Boffetta et al. ( 1) repeats many familiar warnings about reporting false-positive results from epide-miological studies and calls for increased skepticism when assessing and presenting new results. As most epidemiologists know, cautious skepticism is a hallmark of the scientific approach; indeed, it is sometimes said that epidemiologists have a national flower — the hedge. The authors give examples of what they call false-positive results and summarize studies on serum dichlorodiphenyldichlo-roethylene (DDE) and breast cancer in their table 1 and depict the temporal trend of risk estimates studies by year of publica-tion in fi gure 1. Although it is not obvious from the DDE concentrations in table 1, the levels have declined in the United States during the period in which the blood samples were collected. This is a more parsimonious explanation of the trend seen in the fi gure than the elaborate explanation offered by the authors. Furthermore, the authors dismiss the important fi ndings regarding age of expo-sure by saying they were “based on sub-group analyses.” Later, the authors speculate that a “cause of false-positive reporting in cancer epidemiology is publication or reporting bias. ” They illustrate this problem with a funnel plot (fi gure 3) and an associated statistical test. Most of the evidence for the existence of this particular bias appears to come from studies of pharmaceutical products. The opposite problem is more likely in occupational and environmental cancer studies because there is a clear incentive for epidemiologists who are employed by industries to publish small negative studies of the potential hazards that their workers face. For example, in the 1980s, a number of negative studies of soft tissue sarcoma among workers who were exposed to phenoxy acid herbicides and their precursors were published by researchers working for the chemical industry. When the National Institute for Occupational Safety and Health, using their legal rights of access to the industries, combined the data from several differen