x control chart控制图问题请教

Xbar chart
Data plotted        xi
Center line        The center line represents the process average. The default chart usesXbarbar to estimate m.
Lower control limit        The lower control limit (LCL) is the horizontal reference line drawn a specified number of standard deviations below the center line. The formula is:μ-kσ/ni0.5where:ni = size of the ith subgroupk = number of standard deviations from the center line Minitab uses to draw the line. By default, k =3.m =Xbarbars depends on the method you choose to estimate the standard deviation. For a single X chart, an X and R chart, and an X and S chart, the pooled standard deviation is used  by default.
Upper control limit        The upper control limit (UCL) is the horizontal reference line drawn a specified number of standard deviations above the center line. The formula is:μ+kσ/ni**0.5where:ni = size of the ith subgroupk = number of standard deviations from the center line Minitab uses to draw the line. By default, k =3.m = Xbarbar s depends on the method you choose to estimate the standard deviation. For a single X chart, an X and R chart, and an X and S chart, the pooled standard deviation is used  by default.
上述摘自MINITAB14的帮助文件-METHODS AND FORMULAS       
1----minitab中xbar控制图的上控制线为什么不是μ+3σ，下控制线为什么不是μ-3σ，而分别是μ+kσ/ni**0.5和μ-kσ/ni**0.5
2---minitab中bloodsugar（xbar-s）案例的上下控制线按找上述公司计算时也有误差？为什么？我发现xbar-R的案例也有这个问题。
哪为专家能解答？

[ 本帖最后由 herculeswhlee 于 2008-8-29 16:09 编辑 ]