Control Limit Dialogs
Click the UCL or LCL button on the Specs tab of the Results dialog box to display the Upper Control Limit dialog box or the Lower Control Limit dialog box, respectively.
Sigma times z
With this option, you can specify that the upper or lower control limit be calculated by multiplying z times sigma (traditionally 3 times sigma is the most recommended value). You can use the default value for z, or specify the value by typing the number into the box or use the microscrolls.
Compute from p
With this option, you can specify an exact probability on which to base the upper or lower control limit. Note that the actual control limits will always be based on the respective normal distribution values, i.e., when establishing the control limits (as a multiple of sigma), the p value entered here will be converted to the respective normal z value, which is then applied to the computation of the control limits. You can use the default value for p, or specify the value by typing the number into the box or use the microscrolls. This option is not available for all chart types.