Sidarta, N., Tulaar, A. B., Nasution, A. and Suryanto, S. (2008) “Validity and reliability of Preschool Language Scale 4 for measuring language development in children 48-59 months of age”, Universa Medicina, 27(4), pp. 174–182. doi: 10.18051/UnivMed.2008.v27.174-182.