M. Etropolski, MD, B. Lange, MD, MSc, J. Goldberg, PhD, A. Steup, Dipl.-Stat., and C. Rauschkolb, MD, PhD, “A pooled analysis of patient-specific factors and efficacy and tolerability of tapentadol extended release treatment for moderate to severe chronic pain”, J of Opioid Management, vol. 9, no. 5, pp. 343–356, Sep. 2013.