Carey V.J., Wang Y.-G. (2011) Working covariance model selection for generalized estimating equations. Statistics in Medicine 30:3117–3124.
Fu L., Hao Y., Wang Y.-G. (2018) Working correlation structure selection in generalized estimating equations. Computational Statistics & Data Analysis 33:983–996.
Diggle P.J., Liang K.Y., Zeger S.L. (1994, page 166) Analysis of Longitudinal Data. Clarendon Press.