W. Sitinjak, Z. Lubis, M. B. Sibuea, J. R. Saragih, M. R. Aulia, and F. A. Sibuea, “Market Integration and Price Transmission of Robusta Coffee in North Sumatra Province with the World Market”, Agro. Bali. Agric. J., vol. 9, no. 2, pp. 517–530, Aug. 2026.