Okango , Ayubu Anapapa, et al. “ Analysing Four Factor Second Order Models Using Response Surface Methodology With Application in Germination of Melia Volkensii ”. African Journal of Education,Science and Technology (AJEST), vol. 4, no. 2, Dec. 2017, pp. Pg 71-101, http://ajest.org/index.php/ajest/article/view/159.