Lipsitz, S. R., Fitzmaurice, G. M. and Mohlenberghs, G. 1996.
Goodness-of-fit Tests for Ordinal Response Regression Models. Applied
Statistics, 45: 175-190. Pulkstenis, E. and Robinson, T. J. 2004.
Goodness-of-fit Test for Ordinal Response Regression Models. Statistics in
Medicine, 23: 999-1014. Fagerland, M. W. and Hosmer, D. W. 2013. A
Goodness-of-fit Test for the Proportional Odds Regression Model. Statistics
in Medicine 32(13): 2235-2249.