One of the most critical steps in correlating a FEM to measured modal data, is determining what regions of the model require modification. Once these regions have been identified, a number of different methods can be used to modify the properties of the FEM to better match the measured modal data. The problem of determining what regions of the model are different is often referred to as “error localization,” and is very closely related to the field of “damage detection.” TAMKIT implements methods based on examining and minimizing the residual force vectors. These provide a mode by mode measure of where differences exist. Two stiffness perturbation methods are also implemented. These both generate a modification to the A-set stiffness matrix that will match a set of measured mode shapes. While this modification is typically not physically realizable, it can be examined to determine what regions of the structure require update.