Onu, Obineke Henry, Justin Odadami Ejukwa, and Samuel Nanaka. “A Comparative Study of the Performance of Cubic and Quadratic Models for Heptagonal Spherical Two-Factor Second-Order Designs Based on the Sum of Square Errors”. Faculty of Natural and Applied Sciences Journal of Scientific Innovations 4, no. 1 (March 30, 2023): 140–148. Accessed September 16, 2024. https://fnasjournals.com/index.php/FNAS-JSI/article/view/129.