For that purpose, this paper suggests an evaluation method by comparative analysis to four different ways of calculating yield indexes Spk based on six sigma and control information, of S X chart.  ...  To demonstrate the trueness verification proficiency of this approach, this paper includes an industrial case study to assess the process efficiency in Aden's oil refinery, Yemen.  ...  Also the authors extend their gratitude to the Editor and two anonymous referees for their helpful suggestions and careful reading, which greatly enhanced the presentation of this article.  ...