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Cpk Charts Help

The Cpk chart shows how capable a process is of meeting specifications. The Cpk calculations assumes that your process is in statistical control, and that the data is normally distributed. For non-normal data, see “Non-Normal Ppk.” The Cpk chart can be easily updated with new data. This page shows you how to make a Cpk using the data that can be downloadedat this link. This page contains the following:

Data Entry

The data are entered into a worksheet as shown below (using the first part of the data into the example workbook). The data does not have to start in A1. It can be anywhere on the spreadsheet.

The data can be in either columns or rows. Note: if the data are in columns, the program will assume that the number of columns is equal to the subgroup size. If the data are in three columns, a subgroup size of three is assumed in the calculations by the program. Likewise for rows.

Creating a New Cpk Chart

Options for the Cpk Chart

On the input screen for the Cpk chart is the button labeled “Show Options”. If you select that button, the input screen will show the options for the Cpk Chart. Each option is described below. It is not required to select any of these options.

Plot & Sigma Opts, Confidence Limits

Selecting this option displays the input screen below.

Titles, Dates, Target

Selecting this option displays the input screen below.

Cpk Statistics

Selects which statistics to print out in the statistical data box on the process capability chart:

There is an option to check all or uncheck all. There is also the option to set this selection statistics as the default options for future process capability charts.

Historical Values, Outliers

Selecting this option displays the input screen below.

Transform Data

You have the option to transform the data either with the Box-Cox or Johnson Transformations. Selecting this option displays the input screens shown below. If you select the “Transform the Data” option, you will have the option to select either the Box-Cox or the Johnson Transformations. Selecting the Box-Cox option displays this input screen.

Selecting the Johnson Transformation displays the input screen below.

Classes, Minimum

If you select the “Classes, Minimum” option, the following input screen will appear:

Updating the Cpk Chart with New Data

The Cpk Chart can be easily updated with new data after it has been added to the spreadsheet. Select the “Update Charts” on the Updating/Options panel on the SPC for Excel Ribbon.

Then select the chart you want to update. See Updating Charts for more information.

How SPC for Excel Finds New Data to Update the Cpk Chart

When a chart is updated, the program will search for new data based on the option selected for the data being in columns or rows (see the first input screen above). The figure below shows the data from the example workbook. The data range for the Cpk Chart was from B10 (see first figure above) to B40. Two data points have been added in B41 and B42. The chart is then updated.

This is how the program looks for new data:

The process works similarly if the data are in rows.

If you have empty cells in the data, consider putting a space in the empty cell or a character like N/A. The program ignores these types of entries in the calculations and the empty cells do not prevent the program from finding all the data.

Changing the Options for the Cpk Chart

You can change the current options for a Cpk Chart (e.g., adding +/- 3 sigma lines) 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.

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