1. Abd Ghafar, M. N. (1999). Penyelidikan pendidikan [Education research]. Skudai, Johor: Penerbit Universiti Teknologi Malaysia.
2. Ahmad, J. (2002). Validity, reliability and effectiveness of self advancement program module on achievement motivation among high school students in Selangor. Unpublished doctoral dissertation. Serdang: University of Putra Malaysia.
3. Ambo, N., & Siew, N. M. (2017). Development and validation of scientific creativity test through sketches among fifth graders. Sabah Zone Proceedings of National Education Research Seminar, 2017 (pp. 335-345). Tawau: Tawau Campus Teacher Education Institute.
4. Boone, H. N., & Boone, D. A. (2012). Analyzing likert data. Journal of Extension, 50 (2), 1-5.
5. Buck Institute for Education. (2018). Why PBL? Retrieved August 20, 2018, from http://www.bie.org/about/why_pbl.