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| Contents | Index Stepped Control Limits dialog box The Change Limits button in the Analysis Options dialog box opens the Stepped Control Limits dialog box, where you can specify control regions (each with a unique center line and control limit), and the subgroups used to calculate center lines and control limits within each region. The list box at the left will contain all the control regions specified for the data file. These regions result in what is sometimes called stepped control limits, since on the control chart, the control limits and the center line will vary from one region to the next. This is used to signify a process shift. For each region listed on the left, you have several choices as to how the center lines and control limits are calculated. You can use all the subgroups in the region (the default), a range of groups (such as for setting control based on an initial capability study), or from user defined control limit values (also sometimes used for initial capability studies). To add a region To create a region, click on the Add button, and enter the subgroups to be contained in this region into the Control Limit Region dialog box. For each region you can specify the method by which control limits will be calculated. The default setting is to use all the subgroups in the specified region to calculate the control limits and center lines of the control charts. To select one of the other methods for setting control limits, simply choose the region in the left list box, then click on the radio button corresponding to the method you desire. Specify limits based on a range of groups in the region If you wish to see the control limits for the entire data set calculated based on the values for a certain range of data points, select the Groups from radio button. Either place your mouse cursor into the first text box and type the subgroup number, or use the scroll buttons to indicate the first subgroup in the region to be included in the calculation. (Note that you have already defined the region in the list box at left). Then tab over to the second text box and enter the last subgroup in the region to be included in the calculations. Specify limits based on user defined limits If you wish, you may set the upper control limit (UCL) and the lower control limit (LCL) yourself instead of having the software compute them. If you would like the upper and control limits to be computed automatically, leave these boxes blank. User UCL and User LCL: Enter the control limits for the averages or individuals chart, based on the subgroup size specified in the Column Format dialog box. These entries will be used to compute the remaining control limits and center lines. The control limits you enter will be applied to complete subgroups. The software will correctly adjust the center lines and control limits for partial groups. When user limits are entered, output displays the message "Limits were set by user." You must enter both the upper and lower control limits if you wish to set the limits yourself. |
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