The inelastic displacement ratio, CR, is defined as the maximum lateral inelastic displacement demand, inelastic, divided by the maximum lateral elastic displacement demand, elastic, on systems with the same mass and initial stiffness when subjected to the same earthquake ground motion , In both cases displacements are relative to the ground.
Miranda E. and Ruiz J. "Evaluation of approximate methods to estimate maximum inelastic displacement demands". EARTHQUAKE ENGINEERING AND STRUCTURAL DYNAMICS. 2002