A new technique for selecting manipulated variables from a range of possible control inputs is introduced. The new technique, Single-Input Effectiveness (SIE), provides a screening criterion for eliminating certain options when deciding which inputs to the process should be used in a control scheme. These inputs are selected so as to have the largest effect on the selected outputs. The new tool can be used for pre-screening inputs and avoids the combinatorial difficulty which usually occurs when performing control structure selection. Hence for a complex system with a large number of possible inputs, the possible options can be selected efficiently. It has also been proven that the SIE has a close relationship with the non-square RGA.