What is the process capability index and how can you use it in Lean Six Sigma?

1 What is the process capability index?

The process capability index is a ratio of the specification range to the process variation. The specification range is the difference between the upper and lower limits of what your customers expect or accept from your product or service. The process variation is the spread of the data around the average or mean of your process. The process capability index tells you how many standard deviations of your process fit within the specification range. The higher the index, the more capable your process is of meeting the specifications.

2 How to calculate the process capability index?

There are two common ways to calculate the process capability index: Cp and Cpk. Cp is the simplest form of the index, and it is calculated by dividing the specification range by six times the standard deviation of the process. Cp assumes that your process is perfectly centered within the specification range, meaning that the mean of your process is equal to the midpoint of the specification range. However, this is not always the case in reality, and your process may have some bias or shift from the center. This is where Cpk comes in handy.

Cpk is a more realistic form of the index, and it takes into account the actual position of your process mean relative to the specification range. Cpk is calculated by finding the minimum of two ratios: the difference between the upper limit and the process mean divided by three times the standard deviation, and the difference between the process mean and the lower limit divided by three times the standard deviation. Cpk tells you how close your process mean is to either limit, and how much room you have for improvement.

3 How to use the process capability index in Lean Six Sigma?

The process capability index is an essential tool in Lean Six Sigma projects, as it helps to assess the current state of a process, identify gaps and opportunities for improvement, and measure the impact of changes. In the Define phase, the index is used to define the voice of the customer (VOC) and voice of the process (VOP), which can be compared to determine if a process is capable or not. In the Measure phase, it is used to measure baseline capability, validate a measurement system, and monitor variation and mean. During the Analyze phase, it is utilized to identify root causes and key input variables that affect output. In the Improve phase, it can be used to evaluate potential solutions and implement changes. Finally, in the Control phase, it can be used to control an improved process, update documentation, and standardize best practices.

4 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?