Median Chart Help
The Median 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. 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. To use a median chart, you need frequent data and a method of rationally subgrouping the data.
This page shows you how to make the Median chart. The data can be downloaded at this link. This page contains the following:
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 Chart
- 1. Select the data on the worksheet to be included in the analysis. This is the shaded area shown below. You can use “Select Cells” in the “Utilities” panel of the SPC for Excel ribbon to quickly select the cells.
- 2. Select “Variable” from the “Control Charts” panel on the SPC for Excel ribbon.
- 3. The input screen for variable control charts is displayed.
- 4. Select “Median Chart” from the “Subgroup Charts” tab. Select OK.
- 5. The input screen for the chart is then displayed. The program sets the initial subgroup identifiers and data ranges as well as the subgroup size based on the range you selected on the worksheet. This is why there is an advantage to selecting the subgroup and data ranges prior to making the chart. You can edit the ranges if needed here. Enter a name for the chart. In this example, the name “Weight” is used as the name of the chart. This is the name that will appear on the worksheet tab containing the chart. It must be unique – there can not be another workbook tab with that name.
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:
- Subgroup Identifiers: the worksheet range containing the subgroup identifiers
- Data: the worksheet range containing the data in subgroups
- Name of Chart: chart name – must be a unique name in the workbook and is limited to 25 characters
- Subgroup Size: enter the subgroup size; default value is determined by the range selected on the worksheet; maximum subgroup size is 15 (note: if you change the subgroup size here, the software will adjust the data range)
- Subgroup Identifiers: this information is used to search for new data when updating the chart (see below)
- In One Column: the subgroup identifiers are in one column (as in this example)
- In One Row: the subgroup identifiers are in one row
- Automatic Updating of Limits: option controls whether the average and control limits are updated as new data are added, default value is Yes; set this to No to stop the average and control limits from updating
- Show Options: selecting this button shows the various options for this chart (see options below)
- Out of Control Tests
- Control Limit Options
- Titles and Formats
- Manual Control Limits
- Chart Location
- Box-Cox Transformation
- Process Capability
- Select OK to create the chart
- Select Cancel to exit the software
- 6. Once you have select the options you want (again, no options are required to make the chart), select OK and the chart will be generated. An example based on the data above is shown below.
Options for the Median 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:
- Point beyond the control limits: out of control if a point that is above the upper control or below the lower control
- k points in a row trending up or down: out of control if there are k points in a row trending upward or trending downward; two points in a row that are equal break this trend
- k points in a row in Zone C or beyond (one side of avg): out of control of there is a run of k points in a row above or below the average
- Overcontrol: k points in row alternating up and down: out of control if k points in a row are such that the values alternate up and down
- Set Selected Tests as Defaults: this will set the selected test and k values as defaults for this type of chart
- Reset Tests to Program Defaults: resets the tests to the original program defaults for the type of chart.
Control Limit Options
If you select the “Control Limit Options” button, the following is displayed.
The options include:
- Estimate of Sigma: defines the way the software estimates sigma (standard deviation)
- Avg. Subgroup Range (Rbar): estimates sigma from the average subgroup range
- Pooled Standard Deviation: estimates sigma from the pooled standard deviation
- Based Control Limits on +/- ___ Sigma: defines how to calculate the control limits; default is +/- 3 sigma
- Additional Lines for Median Chart: the software allows you to add two additional lines to the chart based on multiple of sigma; additional lines are added above the average and below the average; this option will override the option of putting the 1 and 2 sigma lines shown on the Out of Control Tests option.
- Use Actual Subgroup Size: option to use actual subgroup size to determine control limits if subgroup size varies; default is checked; if not checked, the software uses the subgroup size entered on the chart input screen.
- Check for Trends: option to check for trends; if present, a trend chart will be made using the best-fit line
- Target for Average: option to enter a target for the average
- Select OK to use selected options
- Select Cancel to revert to previous options
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:
- Median Chart Title & Y Axis Label:
- Title: title at the top of the median chart; default is “Median Chart”
- Y-Axis Label: label to put on the median chart vertical axis; default is “Subgroup Median”
- X-Axis Label: label for the horizontal axis for both the median chart and the range chart; default is “Subgroup Number”
- R Chart Title & Y Axis Label:
- Title: title at the top of the range chart; default is “R Chart”
- Y-Axis Label: label to put on the range chart vertical axis; default is “Subgroup Range”
- Rounding to Use for Average and Limits on Chart: used to control the number of decimal places in the values of the average and control limits printed on the chart; default is estimated by the software
- Allow Values Below 0?: select Yes to allow values below 0 on the chart; select No to keep the chart from showing anything below 0; default is Yes
- Print Average/Limits:
- On Avg. and Limits: prints the values of the average and control limits on the average and control limit lines on the chart (this is the default)
- In Chart Title: prints the values of the average and control limits in the chart title
- Dates of Data Collection: enter the start and end dates of data collection; optional; dates will be plotted at the bottom of the chart
- Show Last K Points Only: option to show only the last K points of the chart; calculations include all the data but only the last K points will be shown
- Select OK to use selected options
- Select Cancel to revert to previous options
Manual Control Limits
If you select the “Manual Control Limits” button, the following screen is displayed:
The options include:
- Enter Average and Sigma: the software uses these input values instead of those calculated by the software
- Average: enter the average to be used in the calculations
- Sigma: enter the standard deviation to be used in the calculations
- Enter Median Chart Limits: if you select this option, you must enter at least one of the UCL, centerline, and LCL; the software will calculate the values that are not entered
- UCL: enter the upper control limit
- Center Line: enter the center line value
- LCL: enter the lower control limit
- Additional Lines on Median Chart: the software allows you to plot two other lines with manual entry; lines are plotted above and below the center line; this option overrides the options above to plot lines at +/- x sigma.
- Select OK to use selected options
- Select Cancel to revert to previous options
Chart Location
If you select the “Chart Location” button, the following screen is displayed.
The options are:
- New Chart Sheet: places the chart on a new chart sheet
- In Worksheet with Data: places the chart on the worksheet containing the data
- In New Worksheet: places the chart on a new worksheet
- Select OK to use selected options
- Select Cancel to revert to previous options
Notes:
- Once a chart is made, you can move the chart to new locations in the workbook. The software will find the chart when updating.
- This option is not available when a chart’s options are updated since you can manually move the charts to new locations in the workbook
Box-Cox Transformation
Selecting the “Box-Cox Transformation” button displays the following screen:
The options are:
- Use Box-Cox to Transform the Data: selecting this option reveals the bottom part of the input form
- Options for Lambda
- Find optimal lambda between: finds the value of lambda that minimizes the standard deviation between the default values of -5 and 5
- Find rounded lambda between: finds the rounded value of lambda that minimizes the standard deviation between the default values of -5 and 5; rounded value of lambda are -5, -4, -3, -2, -1, -.5, 0, .5, 1, 2, 3, 4 and 5
- Enter a lambda between -5 and 5: enter a value of lambda to use between the values of -5 and 5
- Calculate lambda for each split limit: this option will calculate the value of lambda for each set of split control limits
- Select OK to use selected options
- Select Cancel to revert to previous options
Notes:
- The default values of -5 and 5 can be changes when finding the optimal or rounded values of lambda, but must stay between -5 and 5.
- The standard deviation that is minimized is based on the option you selected to estimate sigma for the chart (see Control Limit Options)
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. 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:
- Include Process Capability Analysis with this Control Chart: select this to activate the option, the rest of input screen is then shown
- Name of Chart: name of worksheet tab that will contain the Cpk chart; this name cannot be manually changed
- Specifications: enter specifications or nominal; at least one specification is required
- LSL: enter lower specification
- Nominal: enter nominal
- USL: enter upper specification limit
- Sigma & Avg Lines, Shading: select this to show the Cpk chart options (see the Sigma, Average, Shade option for Cpk charts for details, note: the option to change how sigma is estimated is not available; this is determined by the chart)
- Titles, Labels, Target, Classes: select this option to show the titles options (see the Titles, Rounding, Dates, Target, Classes option for Cpk charts for details, note: the option for dates and rounding are not available; these are determined by the chart)
- Capability Results Table: selecting this option places a summary of the process capability results into a summary worksheet; this worksheet contains all the process capability results for the workbook if this option was selected; default value is No.
- Select OK to use selected options
- Select Cancel to revert to previous options
Changing the Options for the Median 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.