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Non-Normal X-mR Chart Help

Control charts are very robust to the type of distribution the process has. However, there may be times you wish to fit your distribution and use that distribution to define the control limits. The SPC for Excel software does this for charts with individual values. This page shows you how to make the non-normal X-mR control chart. The data can be downloaded at this link. This page contains the following:

Data Entry

The data below will be used. The data were generated using a random number generator for a Gamma Distribution with the shape parameter = 3 and the scale parameter = 2. You enter the data into a worksheet as shown below. The data can anywhere on the worksheet and can be in columns (sample identifiers in one column) or in rows (sample identifiers in one row).

Creating a New Non-Normal X-mR Chart

This is all that is needed to make the non-normal X-mR chart. If you select OK at this point, the software will generate the control chart using the default options. Simple and quick.

The options are:

Distribution Options for the Non-Normal X Chart

In the input screen above, there is a section titled “Select Distribution”. Selecting the arrow shows the available non-normal distributions:

Select the distribution you want to use. If you are not sure which distribution fits best, you can run “Distribution Fitting” from the SPC for Excel ribbon. You can either choose to enter the parameters (if you have determined the parameters from distribution fitting) or let the program calculate the parameters. To enter the parameters, select the distribution you want (Gamma in this example) and select the “Enter Parameters” button. The screen below is displayed.

The name of the selected distribution is shown at the top. Select Enter Parameters and the parameters available for that distribution can be entered (shape and scale for the Gamma distribution). If you select Cancel, all parameters are cleared.

Options for the Non-Normal X-mR chart

On the input screen for the non-normal X-mR chart, there is a button labeled “Show Options”. If you select that button, the input screen will show the options available for the chart. It is not required to select any of these options.

Each option is described below.

Out of Control Tests

If you select the Out of Control Tests, the input screen, Tests for Out of Control Points, is displayed below.

These options include only one test for the X chart:

There are four tests for the mR chart:

Titles and Formats

If you select the “Title and Formats” button, the following screen is displayed:

The options include:

Chart Location

If you select the “Chart Location” button, the following screen is displayed.

The options are:


Updating the Non-Normal X-mR Chart with New Data

The non-normal X-mR chart can be easily updated with new data after it has been added to the spreadsheet.  Please see Updating Control Charts with New Data to see how the software finds the new data and updates the chart. 

Changing the Options for the Non-Normal X-mR Chart

You can change the current options for a non-normal X-mR chart (e.g., changing the distribution) by selecting “Options” on the Updating/Options panel on the SPC for Excel Ribbon.

The list of available charts in the workbook will be displayed. Select the chart you want to change options for. The input screen for that chart will be shown and changes can be made. See Changing Chart Options for more information

In addition, once a control chart is made, there are numerous actions you can take on the chart including splitting control limits, removing points from the calculations, adding comments, selecting the range on which to base control limits, etc. Please see Control Chart Actions for details.

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