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Deviation variables have been utilized to simplify the analysis and synthesis of observer-based multivariable servomechanisms. It has been shown that internal stability and output regulation are dictated by the asymptotic behavior of the deviation variables. Similar characterizations are unavailable in the compensator-based approach to servomechanism design. In this note, we introduce a set of deviation variables for the analysis and synthesis of compensator-based servomechanisms.