A new measure of process operability has been developed by the authors. This steady-state, multivariable, and nonlinear measure-called the operability index, or OI-evaluates the effectiveness of a given design by quantifying the ability of the process inputs to drive the outputs over the desired output space while rejecting expected disturbances. The 01 is independent of the inventory control structure, a very important property. This property enables the 01 to be applied during the process synthesis phase prior to control structure selection, thus permitting the comparison of operability for competing designs. This paper provides proof of the property of inventory control structure independence for linear process models (low and high dimension, square, more inputs than outputs, and more outputs than inputs) and the more general nonlinear processes.