Hamzah, Shahidah, et al. “A Study on the Effect of Stroop Test on the Formation of Students Discipline by Using the Heart Rate Variability (HRV) Technique”. International Journal of Engineering & Technology, vol. 7, no. 3.25, Aug. 2018, pp. 640-3, https://doi.org/10.14419/ijet.v7i3.25.17815.