Making Sense of Process Capability

Posted by on Nov 10, 2019 in Continuous Improvement, Process Capability | 3 comments

Process capability is the inherent variability that a process exhibits due to the variability in the process’ inputs.  To better understand what this means, let’s take a look at a simple IPO (Input, Process, Output) diagram for making a morning pot of coffee.

Our output for this process is a strong, bold cup of coffee.  We designate the output of our process as the Y and it can be associated with the Critical to Quality (CTQ) characteristic that our customer wants.  The inputs shown on the left and are called the X’s and are those things needed by the process to deliver the required output.  The inputs I’ve included are the obvious, i.e., coffee, the coffee maker itself, water, coffee filter, and the person making the coffee.

 

But let’s take this example a step further.  Can the type or brand of coffee make a difference in how strong or bold it tastes?  Does the amount of coffee added make a difference in taste?  For instance, my wife and I use a measuring cup and add three scoops to our twelve cup coffee maker in the morning.  My wife has a tendency to level her scoops off while on the other hand I tend to heap my scoops up.  Does the additional coffee my heaped up scoops add make a difference in how our coffee tastes?  Does the amount of water we pour in make a difference?  What about the paper filter?  How about the person making the coffee, either myself or my wife?  What if we allowed our seven year-old granddaughter to make the coffee, would that make a difference in how bold or strong the coffee tastes?

 

We can associate the process inputs or X’s to the 6M’s.  The M’s are: Man (person), Machine (equipment), Material, Method, Measurement, and Mother Nature (environment).  They are contributors to the variation that exists in every process and affect the capability of our processes.    The strength and boldness of coffee will be affected if we vary the type or brand of the coffee, the quantity scooped from the coffee container, the amount of water added, the person making the coffee, i.e., my wife, granddaughter or myself.

 

The formula for process capability is:

 

We measure the variability of our process output (our Y) by calculating the standard deviation, which is shown by the symbol, σ, in the denominator of our equation. The standard deviation is a statistic that measures the variation of our data relative to its mean.  If the data points are further from the mean, there is a higher deviation (more variation) within the data set; thus, the more spread out our data, the higher the standard deviation.

 

The bigger Cp gets, the more capable our process becomes.  There are only two ways to make Cp larger, that is by increasing our specification width, i.e., Upper Specification Limit – Lower Specification Limit (USL – LSL) shown in the equation numerator, or by reducing the standard deviation, σ shown in the denominator.

 

That’s why it’s so important to analyze the 6M’s in your processes.  The more consistent they are (small variation) the higher your process capability will be.  It’s important then that you look at how well people are trained, the method they use, whether they use standard work, work instructions and procedures, and how often you conduct process audits to evaluate whether employees are following them.  That measurement systems are capable and accurate, that machines and equipment are capable and are performing as intended and that material meets specification and do not vary lot to lot or from one shipment to another.

 

Wrapping this up, I’d like to make a couple more points.  In the diagram at the top, you see a formula, Y = F(x).  Some of you may recall learning that relationship in high-school algebra.  It states that the dependent variable, Y, is a function of the independent variable, x.  In process improvement terms it means that our output Y is related to the variation in our x’s which are the inputs, which now we can relate to the 6M’s.  It’s our job in process improvement to understand which of the X’s or M’s has the most influence on our output and be able to control them, i.e., make them more consistent and thus make our processes more capable.

3 Responses to “Making Sense of Process Capability”

  1. Nick Hatzis says:

    Jim,

    Thank you for this fantastic example-focused material for novices to understand process capability! I think most people will relate to this example process. This may be the explanation that gets top management on board with pursuing process improvements.

    • Nick,

      Thanks for the nice review and feedback!

    • Martin Ulrich says:

      Your definition of process capability is well written. This is the kind of training that everyone in the organization needs. I have a six Sigma Black belt certification and I understand the advanced statistical analysis of process variation. But this means nothing to the people on the floor who are actually performing the value-added activities. Educating everyone in the organization to understand the basic concepts of the terminologies being used builds and creates an improvement culture based on a common language.

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