For example, here sample size is huge, USA population gets significant increased risk, while EU population does not. Mixing the two together would result in a smaller but still significant increased risk.
Given the size, it's quite clear that USA population has many other confounding factors that cannot be eliminated by mathematics alone (there is no control).
http://jaoa.org/article.aspx?articleid=2517494
For example, here sample size is huge, USA population gets significant increased risk, while EU population does not. Mixing the two together would result in a smaller but still significant increased risk.
Given the size, it's quite clear that USA population has many other confounding factors that cannot be eliminated by mathematics alone (there is no control).