How Response Factor reported is computed?
ETABS and SAP2000 compute the Response Factor using the following equation:
RESP-FACTOR = [2.31 – 0.41 log (100 (Case + Link damping express in ratio))]/ [2.31 – 0.41 log (100 (Function damping express in ratio))]
The above equation is adopted from Earthquake Spectra and Design by N. M. Newmark and W. J. Hall (published by Earthquake Engineering Research Institute in 1982). You can also find similar equation in ATC 40, Chapter 8 Nonlinear Analysis Procedures.