Bahçecitapar, M., Özge . Karadag, and S. Aktas. “Specification of Variance-Covariance Structure in Bivariate Mixed Model for Unequally Time-Spaced Longitudinal Data”. International Journal of Statistics in Medical Research, vol. 4, no. 4, Nov. 2015, pp. 370-7, doi:10.6000/1929-6029.2015.04.04.6.