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Moving Range (mR) Chart Help

The moving range (mR) control chart is the “mR” part of the X-mR control chart. There are times when you might want to just look at the variation between consecutive samples. The conditions for using the mR control chart are the same as for the X-mR chart, i.e., limited data – only one point to represent the situation at a time. For example, suppose you company is tracking accounts receivable each month. You have limited data – one data point a month. You can use the mR control chart in these situations. You plot the range between consecutive monthly result on the mR chart. For more information on X-mR charts please see our SPC Knowledge Base on our website.

Data Entry

The data for monthly account receivables are available. 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 mR Chart

This is all that is needed to make the 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:

Options for the mR Chart

On the input screen for the 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:

Control Limit Options

If you select the “Control Limit Options” button, the following is displayed. Note that sigma is estimated from the moving range.

The options include:

Titles and Formats

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

The options include:

Manual Control Limits

If you select the “Manual Control Limits” 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:


Box-Cox Transformation

Selecting the “Box-Cox Transformation” button displays the following screen:

The options are:


Process Capability

The “Process Capability” button gives you the option to “tie” a control chart to a process capability chart. This is different from running a process capability option by itself. When the process capability is tied to a control chart, the estimated sigma is based on the option you selected to estimate the value of sigma for the range chart for that control chart. For example, you might have 20 subgroups and split the control limits for the last 10 subgroups. In that case, the process capability (when it is tied to a control chart) will be based on the estimated sigma for the last ten subgroups. When you run a process capability by itself, it would use all the 20 subgroups to estimate sigma. If you manually enter the value of sigma to use for the control chart (see Manual Control Limits above), the software will use that value of sigma for the process capability analysis. The process capability chart is added to a new chart sheet.

If you select the “Process Capability” option, the following input screen is shown. You must enter a name for the control chart before this input screen will show. In the example below, the name of the control chart is Accounts Receivable. The name of the process capability chart will be the name of the control chart plus “Cpk” – or Accounts ReceivableCpk in this example.

The options include:

Updating the mR Chart with New Data

The 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 mR Chart

You can change the current options for a mR chart (e.g., adding a Box-Cox transformation) 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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