Qawqzeh, Y., L. Kayed, M. Otoom, M. Alharbi, and F. A. Khan. “The Investigating of Photoplethysmogram Waveform Amplitude Changes: Aging and Atherosclerosis”. International Journal of Sciences: Basic and Applied Research (IJSBAR), vol. 30, no. 3, Nov. 2016, pp. 126-33, https://gssrr.org/index.php/JournalOfBasicAndApplied/article/view/6555.