How to calculate process capability is described in this post with an accompanying video.
Described is how traditional process capability reporting methodologies have issues and a better way to answer the question “how to calculate process capability.”
Process Capability Indices Equations
The equations for calculating the process capability indices in a Process Capability Report are:
What are your thoughts about the traditional reporting of process capability indices from Minitab and elsewhere?
A typical response to this question is that they are confusing, and I don’t know what to do about the reported index values.
Traditional How to Calculate Process Capability Reporting Methodology
The values in a process capability report are a function of how the data are obtained, e.g.,
- Situation 1. An X-bar and R control chart with subgroups of a sample size of 5.
- Situation 2. An X (Individuals) control chart with individual measurements.
- Situation 3. A random sample of measurements from a population.
A process can have very different reported capability indices values depending on how measurements were taken from the process, e.g., Situation 1, 2, or 3.
In process capability indices equations, the standard deviation depends on how samples are selected, as illustrated in the following tree diagram, where “Short term” refers to Cp and Cpk. In contrast, “Long-term” refers to Pp and Ppk.
Assumptions: Process Capability Reports
- For process capability indices to be valid, data must be from a stable process.
- One problem is that the process control chart that assesses process stability is in a separate report-out than its process capability/performance statement.
- Because of this, one would not know from a process capability report if the process was stable or not.
- The previous process capability equations are appropriate for normally distributed data.
- Customers typically ask for their purchased product to meet a process capability (Cp and Cpk) requirement; however, this “short-term” measurement does not reflect customer needs. Process performance (Pp and Ppk) “long term” is a more appropriate metric for addressing customer needs.
Conclusion: There is much confusion and potential problems with Cp, Cpk, Pp, and Ppk reports.
30,000-foot-level Reports Address Traditional Process Capability Reports Shortcomings.
30,000-foot-level reporting overcomes all these traditional “Process Capability Reports” shortcomings and more
I will use Example 12.2 from Integrated Enterprise Excellence Volume III to illustrate the benefits of 30,000-foot-level reporting over a traditional process capability/performance analysis report, as shown in the following Minitab report.
I will first show and describe the results from a 30,000-foot-level free app alternative report-out and then show how you can access and use this app for your dataset.
Shown in the following figure is the Example 12.2 dataset and its traditional Minitab Process Capability Report-out for a specification of 72-78
A 30,000-foot-level report for this dataset is:
This 30,000-foot-level report consists of three components:
- The individuals chart on the left of the report assesses process stability. If no points are beyond the UCL and LCL (Upper Control Limit and Lower Control Limit) lines of the individuals chart, the process is considered stable (and predictable)
- The second component is a normal-probability plot of the raw data from the individuals chart.
- If the data follow a straight line, the normal distribution is considered a good model for the data and can be used to make a valid process capability statement
- Since there was a specification of 72 and 78, the percentage values from these values are shown on the y-axis.
- The percentages above and below these y-axis values are used to determine the non-conformance percentage rate beyond the product’s specification limits.
- The third component is the statement at the bottom of the report, which reflects the non-conformance percentage rate determined from the probability plot. If a prediction statement is undesirable, there is a need for process improvement.
Benefits of 30,000-foot-level reporting over traditional process capability reports (for how to calculate process capability better) include:
- Process stability and its capability are provided (if the process is stable) in one report-out
- A reported estimated futuristic non-conformance rate that is easier to understand than Cp, Cpk, Pp, & Ppk
- Chart creation inputs are available for non-normal data situations
- Provides an output statement from a high-level point of view (i.e., a customer-needs point of view)
- If a 30,000-foot-level predictive process output response statement is unsatisfactory, there is a need for process improvement.
- When a process is enhanced, its 30,000-foot-level individuals chart should be staged to a new, improved level of performance that would be shown in a new probability plot and stated at the bottom of the report-out
You can access this free app through the webpage https://smartersolutions.com/free-app/
After registering and logging into the app, you will see the app’s report-out inputs (In this figure, explanation notes were added.):
The two books shown on the right side of the above figure are written as a novels. Mangement 2.0 is the first book in the two-book series. Leadership system 2.0 is the second book.
Both books are available in paperbook, e-book and audio book formats.
I will now create a 30,000-foot-level report for the viscosity dataset
There are instructions for using this app in the following figure (for how to calculate process capability better)
Additional Information on 30,000-foot-level Reporting And How to Calculate Process Capability (A Better Approach)
Articles
These articles provide, among other things, an informative video about the 30,000-foot-level methodology and its reporting.
- “KPI Management: KPI Metric Reports that lead to the Best Behaviors”
- “Control Charting Techniques Integration with Process Capability and Enhanced KPI Reports”
Books
IEE Volume III has over 100 examples and 250 exercises on organizational measurements and improvements
This 2-book bundle provides solutions to IEE Volume III examples and exercises, including obtaining the datasets spreadsheets.
Application of this Reporting to the Business
The IEE 9-step system provides a vehicle for organizational management that can have automatically updated 30,000-foot-level metrics throughout the business.
Next Steps for How to Calculate Process Capability using a Better Approach
- Schedule a video meeting with me to see how your organization could benefit from 30,000-foot-level reporting using one of your datasets.
- You can schedule a video meeting session with me.
- Through the link. This link is also provided at https://smartersolutions.com.
- By emailing me at [email protected]
- A PDF copy of the video presentation is available through the link: How do you calculate process capability – A better way
- Subscribe to https://www.youtube.com/SmarterSolutions to see more do-it-smarter techniques for metric reporting and enhanced process improvement methodologies, so the big-picture benefits.