You can never control for all variables when humans are involved. (You can only control for variables that your data contains.) EDIT: From the article: "[There

You can never control for all variables when humans are involved. (You can only control for variables that your data contains.) EDIT: From the article: "[There's a] need for future research to incorporate more accurate measures, such as urinary sodium excretion, to more precisely assess the impact of sodium intake as a potential confounding factor."