Kusin, J., H. S. R. Sinaga, J. Khoma, J. M. Houtkooper, and U. Renqvist. “The Preschool Child in Suka Village, North Sumatera”. Paediatrica Indonesiana, Vol. 21, no. 9-10, Oct. 1981, pp. 181-91, doi:10.14238/pi21.9-10.1981.181-91.