Tips and tricks: What if my customer requires Cpk values for our product?

Steve DaumIn addition to the perennial need to reduce costs expressed by manufacturers, many need to provide Cpk values to their customers—a process that demands countless hours of calculation, unless they have SQCpack to do the work for them. If your customer is indeed requiring Cpk data, here are some questions to be asked:

  1. Does the customer need only the number, or will he/she want the charts to support that data?
  2. If charts are needed, will the customer want a histogram, a capability summary chart, a report, a certificate of analysis, a Six-Way Analysis, or something else? What product is involved?
  3. Has the customer given you data, and has that data been entered?
  4. Are there upper and/or lower specification limits?
  5. Does the customer want the Cpk value reported as a number for only one characteristic, or for many?
  6. Is the need to have Cpk values for a specific time period (e.g., first quarter, 2016)?

Moving onto more specific questions about your own Cpk values, you may want to conduct a self-examination with the following questions:

  1. Do we have an adequate amount of time-ordered data?
  2. Have we plotted this data on a control chart?
  3. Does the control chart show a stable process? (i.e., It is in a state of statistical control.)
  4. Is our measurement system adequate for the application?

Once you can answer “Yes” to these four questions, you are taking the right steps needed to determine the capability of your process. To find the capability of your process, the mean and standard deviation need to be calculated. SQCpack will use the data from the process to calculate these two descriptive statistics. The last thing you need is one or both specifications which you will supply. Once this information is available, SQCpack can provide the capability statistics such as Cpk.

Now it is possible to move on, to consider how the capability index (Cpk) can be provided to your customers. Depending how you will share the Cpk values with others, there are many ways to provide this information.

If you are using SQCpack and need to report the Cpk, here are some ways to do it:

  1. Display the Cpk, and possibly other statistics in the title of a control chart.
  2. Display a histogram with the Cpk, and possibly other statistics.
  3. Display a Capability Summary chart.
  4. Display a Six-Way Analysis chart.
  5. Display a Report that includes the indices you need.
  6. Display a Certificate of Analysis.

Since there are a variety of ways to report Cpk, the fundamental need is communication with the customer. Once you have established exactly what that customer wants, and in what format it is to be delivered, sharing information about your own system, including the Cpk, is easy.

2 thoughts on “Tips and tricks: What if my customer requires Cpk values for our product?

    • Surinder – I’m glad to know you found the article helpful! Thanks for reading!

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