ABSTRACT
In our professions, we may sometimes have to deal with issues in which there are two or more variables at a stake. For example, in a company that builds denitrifi cation plants for nitrate elimination, we may wonder to what extent the nitrate level of a river may change when the water pH is modifi ed. And if it does, how does it change? And what other variables may infl uence this change? Can we predict the level of nitrate from a group of variables?
