Akuba, N. H., and N. Risdiana. “Comparison the Level of Standard Deviation of N-N Interval (SDNN) Among Adolescent in Non Smokers and Smokers in Yogyakarta”. JKKI : Jurnal Kedokteran Dan Kesehatan Indonesia, vol. 9, no. 1, Apr. 2018, pp. 30-34, doi:10.20885/JKKI.Vol9.Iss1.art6.