Salihah, P. R., S. Sahiruddin, and P. D. D. Degeng. “An Analysis of Text Readability in 9th Grade English Textbook Using FRE Formula on Readable Software”. Diglossia: Jurnal Kajian Ilmiah Kebahasaan Dan Kesusastraan, vol. 11, no. 1, Sept. 2019, pp. 24-33, doi:10.26594/diglossia.v11i1.1753.