SPC的一些疑问

刚刚接触SPC控制图，有一些疑问：
我司有一道调试工序，有七八个工位，各工位间的精度不同，调测的有一个数据要求是-22，公差要求+-0.3。
OQC对该参数抽检，OQC检验公差要求+-1.5。
对这个参数作SPC控制，是用OQC抽检的样本数据，还是该分工位作单独每个工位的SPC控制图？
我从OQC抽检工位统计了250个数据，作X-S控制图，对于超出控制线的点，该如何进行进一步分析改进？
求高手指点。
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