[FandM]

In an experiment with various solutions and concentrations, as well as calculating rates of the reaction:
what variables are controlled in all trial?

[Kuahji]

Usually the concentration of one reactant is held constant, while the other is varied, at least when you're trying to find out rates of reaction.  This way you can find out how the rate of reaction is effect by each reactant. 

Hopefully I read your question correctly.