∞ generated and posted on 2016.08.27 ∞
Conditions that one intentionally and, often through great effort, keeps from changing between treatments during experiments.
The number of controlled variables approaches infinity. Typically controlled variables are not considered individually, however, but instead are 'controlled' by making sure that all treatment groups within an experiment are treated identically, other than in terms of independent variables.
Exceptions occur when experiments are less easily controlled such as one sees in medicine in which differences among individual subjects is a given, though these too are something that one attempts to minimize in a well-run medical study.