Matajang, Sri Wahyuni, and Erlinda Muslim. “Analysis of Product Quality Improvements to Reduce Coffee Bean Defects With Six Sigma Method”. Jurnal Sistem Teknik Industri 24, no. 1 (January 25, 2022): 107–123. Accessed September 1, 2025. https://talenta.usu.ac.id/jsti/article/view/7517.