Cp, Cpk vs Pp, Ppk

In the last post we looked at the different process capability indices like Cp, Cpk, Cpm etc. But there is also Pp and Ppk indices that measure process performance. There is a fundamental difference between the two types of indices.
We will now try to understand the difference between process capability and process performance.
And when to use what capability index?

Process capability measures the ability of the process to consistently produce output within specification limits. Process capability indices are calculated assuming the process is statistically stable i.e. has only common cause variation.

Process Performance is an estimate of the process capability during initial setup. The indices are calculated assuming the process is unstable. Such a process has both common and special cause variation. A comparison between process capability and process performance helps understand the difference between the two.
 

 
Have you used Cp, Cpk or Pp, Ppk for measuring your process capability? We would love to hear from you at info@sybeq.com.

Histogram & capability analysis

In the last post we looked at tools for analyzing distribution of variable data. Histogram is the most popular distribution analysis tool. Using the histogram we can also predict the capability of the process.

Histogram shows the frequency distribution of the data. The curve shows shape of the distribution. Skew measures asymmetry of the distribution. A positive skew indicates a histogram with a longer right side tail whereas a negative skew has a longer left side tail.

So where is the tie in between the histogram and capability analysis?
Process capability analysis compares process performance against specifications. Histogram shows the distribution of process data. If the histogram graphically showed the specification limits then it is showing if the process is performing within specs or not. So visually the histogram would show if the process is producing good parts or not.

Numerically capability of a process can be analyzed by calculating the capability indices. There are several capability indices each showing an aspect of process capability. Most popular capability indices are Cp, Cpk, Pp, Ppk.

Read our White Paper on different capability indices and their uses.

What capability indices do you to determine process capability? How do you use the histogram for distribution analysis?

Distribution Analysis for variables

Distribution analysis for variable data is one of the critical analysis that help us understand how much
variation exists in the data. This in turn helps us understand how wide the process is and whether it falls within the specification limits.

There are several tools for studying distribution of variable data. The tools listed below are some of the popular ones:
1. Histogram
2. Stem and Leaf plot
3. Dot plot
4. Box plot

Histogram being the most popular to understand the nature of distribution of variable data. Dot and Stem & Leaf plot are more detailed versions for analyzing data distribution.
Box plots (also known as Box and Whisker plot) are used to compare distributions. Example for using a box plot would be to compare machine performance.

The figure below shows a comparison of the 4 tools.

We would love to hear which of the above tools have you used for analyzing distribution of variable data. Write to us at info@sybeq.com

Analytical tools for variable, attribute data

We have looked in the data analysis piece of Lean Six Sigma projects.
By now we know that data analysis is key in a Lean Six Sigma project. At the same time it is equally important to avoid analysis paralysis. A Green or Black Belt must know that there are different analytical tools for the type of data.

There are 2 data types
1. Variable or Continuous : Data that is measured and readings can have decimals. Eg. Weight, length, diameter. 3.56lbs, 4.5mm
2. Attribute or Discrete: Data that is counted and readings are integers. Eg. number of rejects, number or cracks

In attributes, there are 2 categories specific to the type of count.
Attribute types:
1. Defectives - The number of parts that are rejected are defectives. Also called non conforming units.
2. Defects - Number of specific problems with parts eg. burred, cracked, scratched parts. Defects could be fixed by repairing them. These parts are called non conformities.

For every data type there is a set of analytical tools. The figure below shows the commonly used tools by the type of data.

Types of analysis in LSS projects

LSS projects focus on data driven process improvements. Lean Six Sigma is all about data. Every phase of the DMAIC approach in LSS needs to be backed with data. Most of the data is collected in the Measure phase. This data gets analyzed, verified and validated using LSS tools.

Along with data analysis it is important to know about the process. So process analysis plays an equally important role in LSS projects. Knowledge about the current state of the process helps understand what the process steps of the current process. Here it is necessary to use the Go See Lean principle where the LSS team walks the process as it is currently running. Some Green and Black belts make the mistake on relying work instructions or hear say for the current state. However this may not represent what is actually being done on regular basis.

The figure below shows the types of analysis and some useful tools:


Analysis of critical parameters is the key to a successful Lean Six Sigma project. The challenge lies in identifying the critical parameters, gathering data and choosing the right analytical tools. Over and above knowledge about the process plays an important role too.

Have you experienced challenges related to analysis in your LSS project?

How to overcome Analysis-Paralysis in Lean Six Sigma?

We have established the fact that the Measure and Analyze phase of the DMAIC approach are the most challenging. LSS teams spend considerable time in these two phases. To overcome challenges of gathering and analyzing data, we must understand the purpose of the Measure and Analyze phases.

In the Measure phase, the LSS team is trying to establish the process map, baseline for the primary metric used in the project and the amount of process variation. Creating a process map requires the team to actually go see the process and create the map as they walk through the process steps.
Some popular tools for measuring baseline & process variation include:
Histogram, scatter diagram, trend chart for historical data, capability studies and gage R and R studies.

In the Analyze phase the LSS team is trying to determine root cause of the problem and validate it too. Fishbone diagram or Affinity diagram and Potential cause Prioritization matrix are the two tools for identifying root causes. Then comes root cause validation which requires establishing a hypothesis statement and choosing the appropriate statistical test for validating the hypothesis. Test the hypothesis with enough sample data. Most LSS teams struggle here.

The Green or Black Belt leading the LSS project has to ensure the team maintains focus on getting the required information using the necessary statistical tools.

What are some of the tools you have used to overcome analysis-paralysis in LSS projects?

Dangers of Analysis-Paralysis in Lean Six Sigma

Continuous improvement projects require teams to collect data and analyze it.
Lean Six Sigma method needs metrics and data to back up everything. Even the success of the LSS program is measured and tracked.

Sometimes Lean Six Sigma Green and Black belts analyze data more than needed. In LSS projects using the DMAIC approach the Measure phase is where hard data on the primary metric is collected. Preliminary data analysis begins in the Measure phase and continues into the Analyze phase where root causes are identified. Many LSS projects get stuck in the Measure or Analyze phase.

There are several reasons for this. Some of the key reasons are:
1. Data is hard to find/gather
2. There is too much or too little data
3. Incorrect data is collected
4. Green and Black belts over analyze the data
5. Preliminary data analysis does not reveal much so more data is collected

The Measure and Analyze phases of the DMAIC approach are most challenging. These phases take time and effort from the LSS team. The team needs to have the patience and persistance to get through these phases. The team leader (Green or Black belt) needs to get through the data collection and analysis portion efficiently/effectively. Without getting bogged down by data and not getting caught
in analysis-paralysis is the critical aspect to keep the project moving forward.

In our next blog post we will discuss the tools and techniques to overcome Analysis-Paralysis.

Have you experienced Analysis-Paralysis in an improvement project? How have you overcome it?