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Short-Term Process Capability Studies

cp and cpk industrial engineering lean six sigma manufacturing mean normal distribution process process capability process management six-sigma standard deviation statistics
 

Short-term process capability studies are conducted to establish the performance entitlement of a process. This type of capability study captures a snapshot of a process under ideal conditions. The value of establishing process entitlement is that it can be compared it to the actual long-term performance and hence be used to identify improvement opportunities. The idea is that if we can perfect a process for a day then why can’t it be done every day.

Here are the steps to take:

  1. Fine tune and tightly control the manufacturing process. After completing preventative maintenance procedures, dial in the process and let it settle into a steady-state of operation. Your best operator should be running the machine using high quality raw materials. The process needs to operate at its very best.
  2. Collect a minimum of 30 samples.
  3. Have the samples tested by someone qualified.
  4. Use statical software and enter the sample data along with the product spec limits. I recommend stat software such as Minitab or Excel or try the R-Project.org, which is a free online option.
  5. Calculate the Process Capability (Cp and Cpk)
  6. Interpret the capability results. Here’s how:

Cp is a measurement that compares process variation to product specifications. Cpk is a little different than Cp in that Cpk considers statistical mean in its calculation. So, if a process moves off center, the Cpk will change but Cp will not change. As such, always compare Cpk to Cp to determine if your process is running off center in which case, your process isn’t performing optimally. So take the action of centering the process. 

An interesting fact that most people don’t know about Cp and Cpk is that calculations provide a level of statistical forgiveness. You see Cp and Cpk use a rolling format in their calculations of standard deviation, and this removes the negative impact of process shifts from your process capability results.

In general terms, every one standard deviation or sigma of process variation equates to 0.33 Cp. And every two standard deviations equates to 0.66 Cp. And a three sigma process will yield a process capability of 1 Cp. A six-sigma process will reflect a Cp of 2.0.

Remember, a short-term study reflects process entitlement. The next step is to complete a  long-term study because it reflects real-life process performance. Comparing the process entitlement to the actual performance provides us with our process opportunity.

Six-sigma methodology is all about driving out process variation and achieving six-sigma process performance. This in turn reduces process defect rates and improves profits.

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