Generalized estimating equations (Liang, K. Y. and Zeger, S., 1986, Biometrika 73, 13-22) allow longitudinal or clustered data to be modeled with minimal assumptions about their dependence structures.

Journal of the Royal Statistical Society. Series C (Applied Statistics), Vol. 55, No. 4 (2006), pp. 507-524 (18 pages) Generalized estimating equations for correlated repeated ordinal score data are ...

Instead, we develop a multivariate model for ordinal trait analysis and implement an EM algorithm for parameter estimation.