Performance of composite measures used in a trial of etanercept and methotrexate as monotherapy or in combination in psoriatic arthritis by Coates, L, Merola, JF, Mease, PJ, Ogdie, A, Gladman, DD, Strand, V, van Mens, L J.J., Liu, L, Yen, PK, Collier, DH, Kricorian, G, Chung, JB, Helliwell, PS