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# Median-R Chart

The Median-R control chart is seldom used any more. It was popular before calculators and computers were commonplace because there were no calculations required to plot a point on the median chart and just a simple subtraction to plot a point on the range chart. This type of chart does use subgroups like the Xbar-R and Xbar-s control charts. For example, you might be measuring the conversion from a batch reactor. You make three batches a day. You use those three batches to form a subgroup. You determine the median of the subgroup and plot the median on the median chart. You calculate the range (the maximum – minimum) of the three batches and plot the ranges on the R chart. To use a median chart, you need frequent data and a method of rationally subgrouping the data.

### Data Entry

Suppose we monitoring the weight of a product. We take four samples at the start of each hour and use those four samples to form subgroups. You enter the data are entered into a worksheet as shown below The data does not have to start in A1. It can be anywhere on the spreadsheet. The example is using a subgroup size of four. The data can be in rows or in columns.

### Creating a New Median-R Chart

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

The options on the input screen are:

### Options for the Median-R chart

On the input screen above for the 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.

The options include:

Note: the additional lines, check for trends and target for average options apply only to the median chart.

#### 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:

Notes:

#### Box-Cox Transformation

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

The options are:

Notes:

#### Process Capability

The “Process Capability” button gives you the option to “tie” a 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 chart, the estimated sigma is based on the option you selected to estimate the value of sigma for the range chart for that 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 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 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 chart before this input screen will show. In the example below, the name of the chart is Median-R. The name of the process capability chart will be the name of the chart plus “Cpk” – or Median-RCpk in this example.

The options include:

### Updating the Median-R Chart with New Data

The 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 Median-R Chart

You can change the current options for a 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 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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