AL-RABASI, D.A.M. وآخرون 2022. X-bar Chart Based on Six Sigma to Continual Quality Improvement: Implementation on Oil Vapor Pressure Characteristic, at Yemen’s Aden Oil Refinery. مجلة العلوم التربوية و الدراسات الإنسانية. 25 (سبتمبر 2022), 671–699. DOI:https://doi.org/10.55074/hesj.v0i25.549.