1. Panel A shows the situation that would exist if breathing secondhand smoke caused lung cancer. Panel B shows the situation that would exist if, as suggested by a tobacco industry consultant, people exposed to secondhand smoke were more likely to own pet birds and the birds carried diseases that caused lung cancer, while there was no connection between breathing secondhand smoke and lung cancer. Since owning a pet bird would be linked both to exposure to secondhand smoke and lung cancer this (unobserved) confounding variable could make it appear that secondhand smoke caused lung cancer when, in fact, there was no link.
Source: Chapter 2. How to Summarize Data, Primer of Biostatistics, 7e
Citation: Glantz SA. Primer of Biostatistics, 7e; 2012 Available at: Accessed: October 29, 2017
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