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. 4.9, Oct. 2018, pp. 178-81, https://doi.org/10.14419/ijet.v7i4.9.20668.