I don't have time to read the study but if you want to have a look yourself, the terms to look for are "confounder", "confounding factor", ... https://en.wikip

I don't have time to read the study but if you want to have a look yourself, the terms to look for are "confounder", "confounding factor", ... https://en.wikipedia.org/wiki/Confounding https://en.wikipedia.org/wiki/Confounding