John R. Slate, et al. “Differences in Mathematics and Science Achievement by Grade 5 and Grade 8 Student Economic Status: A Multiyear, Statewide Study”. Global Journal of Human-Social Science, vol. 17, no. H5, Mar. 2017, pp. 15-26, https://socialscienceresearch.org/index.php/GJHSS/article/view/2332.