1. Boersma, P., & Weenink, D., (2013). Praat: doing phonetics by computer. 5.3.47 edn. http://www.praat.org/.
2. Browman, C. P., & Goldstein, L., (1995). Gestural syllable position effects in American English. In F. Bell-Berti, & L.J. Raphael, (Eds.), Producing Speech: Contemporary Issues, 19-33.
3. Core Team, R. (2018). R: A language and environment for statistical computing. Vienne, Austria: R Foundation for Statistical Computing.
4. A dialect study of American r's by x-ray motion picture;Delattre;Linguistics,1968
5. English Pronunciation 1500–1700, volume II;Dobson,1957