Quality Management

What is Process Capability? Index, Formula, Example & Everything to Know

Written byContributing Writer
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Updated onJuly 31, 2023

In manufacturing, the key to profitability is consistency. You need to produce products that meet tight guidelines to satisfy customer requirements. Too many defects can undermine quality and credibility. Too much waste or rework can hurt the bottom line or cause you to miss delivery deadlines. Enter the concept of process capability.

It turns out that processes developed for manufacturing decades ago apply to nearly every industry today — from manufacturing to healthcare to software development to any industry today. In other words, understanding and learning how to use the principles of process capability in any professional team or role is advisable.

Let’s explore the concept of process capability, what it is, and how you can learn how to apply it to your existing or new career by earning a Lean Six Sigma certification in an immersive online program.

So, What Is Process Capability?

Process capabilities compare the output of a process to its specification limits. In other words, how often does a process yield the desired results with a range of acceptable outcomes? Measuring process capability can get complex. You must account for the specifications, upper and lower limits, and centering those.

You can think of it this way. Let’s say you are trying to park a vehicle in a one-car garage. If the car is too wide, it won’t fit inside. If the vehicle isn’t centered when you pull in, it won’t fit and may hit the side of the garage, creating a defect. If the car fits within the width and is centered, it fits. Manufacturing processes, and all others, must fit this narrow range and be well-centered relative to the specs.

In short, process capability processes will help you understand the potential for meeting specifications, identify root causes of defects or deviations, and improve overall process output.

Also Read: What Is Lean Six Sigma? A Comprehensive Guide to Understanding the Methodology

What Is a Process Capability Index?

Results are expressed differently, but stakeholders are typically most interested in the process capability index or a process capability ratio. This index is indicated as Cpk, which measures process capability to show how close a particular process can deliver the results to the required specifications.

Another term you will need to know to understand process capability meaning is Cp, which measures the process’s potential capability to meet specifications.

The Difference Between Cp and Cpk

While the difference between Cp and Cpk is subtle, it’s crucial. Cp assesses the process spread relative to tolerances, assuming the process is centered on the target. Cpk considers the spread and centering, determining the capability to meet specifications even if the process could be more perfectly centered. This helps uncover the actual capability.

Advantages of a Capable Process

A capable process has several key advantages, but the biggest is higher quality. When processes are performed within acceptable limits, you will consistently meet standards. This leads to fewer defects and improves customer satisfaction.

Other advantages include:

Cost reductions: Less waste and rework
Operational efficiency: Less time monitoring and troubleshooting processes.
Reduced risk: Reduces risk for poor quality, delays, or failure to meet customer expectations.
Standardization: Capable processes help develop standard procedures to improve quality
Compliance: Increase the likelihood of meeting compliance or regulatory requirements
Reliability: Consistent output creates more reliable forecasting for lead times and delivery

How to Measure and Calculate Process Capability

There are a few steps involved in measuring and calculating process capability. These include:

#1. Data collection

The first step is to collect sample data from your analysis process. This gives you a short-term baseline.

#2. Identify specifications

Determine how much deviation from specifications is acceptable before resulting in a defect. These are defined as upper and lower limits, representing the acceptable range for process outputs.

#3. Calculate process mean

Find the mean of collected data with the upper and lower limits defined. Add the values and divide the sum by the total data points. This shows the typical value the process produces.

#4. Calculate the standard deviation

Next, you need to calculate the standard deviation (σ). This measures the variability (spread) of the output. In other words, the standard deviation shows how much individual values generally deviate from the mean.

#5. Calculate Cp

Use the process capability formula to assess the potential capability to meet specifications, assuming the process is centered on the target value. Here’s the formula for calculating process capability.

Process capability formula Cp = (Upper limit – Lower limit) / 6σ

#6. Calculate Cpk

The process capability index must account for the spread and centering deviations.

Process capability index formula Cpk = min [(Upper limit – Mean) / 3σ, (Mean – Lower Limit) / 3σ ]

#7. Interpreting the Capability Index

With results in hand, you must compare the value of Cp and Cpk to determine process capability. The higher the value, the better the capability.

Cp or Cpk < 1: Process is incapable and requires improvement.
Cp > 1 or Cpk > 1.33: The process is conditionally capable. However, improvements are likely necessary to ensure consistency.
Cp or Cpk > 1.33: The process is considered capable and meets quality requirements.

If the Cp and Cpk are close, the process is well-centered to achieve the target value. If Cpk is lower than Cp, the process could be better centered.

Also Read: What is Lean Methodology?

Process Performance Indices Formula

The process performance indices are Pp and Ppl. They are similar to Cp and Cpk but have key differences. While Cp and Cpk evaluate the process’s ability to meet specifications, Pp and Ppk offer a snapshot of current performance. In other words, Cp and Cpk measure potential over the long term, while Pp and Ppk measure actual process performance and stability during a defined period.

Process performance formula Pp = (Upper limit – Lower limit) / 6σ
Process performance index formula Ppk = min [(Upper limit – Mean) / 3σ, (Mean – Lower Limit) / 3σ ]

Process Capability Example

A process capability study can help uncover deviations outside of tolerances. Let’s look at an example of process capability.

For instance, suppose you are manufacturing parts required to weigh one pound with an acceptable deviation of up to 0.05 ounces. In that case, you can measure how often parts meet these specs to determine whether you have an efficient process. When you find unacceptable discrepancies, you can examine the root cause for deviations by conducting a capability analysis.

Factors Influencing Process Capability

Several factors influence a process’s capability. Controlling these factors is necessary to improve quality and meet the required specifications. While the example below refers to manufacturing, the same principles apply to all industries today.

Factors include:

Machine condition: Old, misaligned, poorly maintained equipment creates variations.
Materials: Inconsistent material properties can lead to deviations in outputs.
Tooling: Defects in molds, dies, or misalignment create inconsistency.
Measurements: The accuracy and repeatability of measurements are crucial.
Methods: Standardization of processes also impacts variations.
Environment: Temperature controls also contribute to process variability.
Production volume: Tooling can degrade and wear faster at high volume.
Production rate: Processes that run too quickly can make it challenging to control critical variables.
Operators: Experienced and well-trained operators will produce less variability.

Controlling these variables is critical to creating a stable and capable process.

What’s The Difference Between Capability and Stability?

Stable processes seldom have outliers or unusual output values. A Capable Process is one in which the observed variation is mainly within the specification limits or, put another way, has minimal rejections. This process is usually produced as per customer requirements.

Long story short, a process operating within its control limits is stable, while one operating within specification limits is capable.

What is PPK?

Ppk is a process performance index that shows how well a system meets specifications. Its calculations use the individuals’ sigma, also called the actual sigma, to indicate how the system is running compared to the specifications.

What’s the Difference Between CP, CPK, and PP, PPK?

We use Cp and Cpk stable, mature processes under statistical control. In contrast, we use Pp and Ppk when analyzing performances in a pre-production run or a new process that still needs statistical control.

Additionally, Cpk tells the user about the process’s future capability. At the same time, Ppk shows how the process was performed in the past. Comparing Cpk and Ppk can help identify particular cause variations.

Process Capability for Non-Normal Data

When working with transactional or service processes, we often work with lead-time data, which typically doesn’t follow the normal distribution. Here are the five steps to conduct process capability using non-normal data.

Collect the data
Identify the distribution shape
Verify the stability
Find which non-normal distribution best fits the data
Perform the non-normal capability analysis

Tips for Improving Process Capability Scores

Here’s a couple of tips you can use to improve process capability scores:

Thoroughly analyze existing processes using control charts
Identify underlying causes of variability
Respond to identified issues quickly and effectively
Regularly take on small projects to build up your expertise in the field
Pay attention to customer feedback and understand their needs

Tools for Process Capability Estimation

Various tools are used in process capability estimation. Computer simulations can model processes to predict capability under multiple conditions. This is helpful considering the long list of factors that can impact capability. Computer models can determine the impact on capability based on tuning or refining individual factors.

Besides Cp, Cpk, Pp, and Ppk, other tools used in process capability include:

Histograms: Visual display of data distribution to assess shape, center, and spread.
Control charts: Xbar-R, Xbar-S, or Individual-Moving Range.
Probability plots: Evaluate data distribution and deviations compared to norms.
Pilot production runs: Short production runs to estimate capability.
Tolerance analysis: Assessing how individual part tolerance compares to overall tolerance.
Process mapping: Using flowcharts to identify each step in a process to look for variations.
Fishbone diagrams: Visually brainstorm and categorize potential causes for deviations.

Another popular method is the Design of Experiments (DOE). DOE is a structured statistical approach to determining all factors influencing a process. Process factors (inputs) are tested at different levels to determine their impact on process outputs. This can provide real-world data to determine whether adjusting key inputs improves performance.

Also Read: Demystifying the Theory of Constraints

Practical Concerns When Conducting Capability Studies

You must also be aware of several practical concerns when conducting capability studies. One of the most critical concerns is the sample size. If you have too small of a sample size, the results may not be adequate to provide valuable data.

Other factors you need to control when conducting capability studies include:

Process conditions: You want to conduct capability studies in a typical production environment under normal operating conditions.
Data handling: Measurement tools must be calibrated, and consistent procedures must be in place for data collection and analysis.
Specifications: Make sure specifications align with customer requirements and are up to date.
Monitoring: As changes are made, such as new operators, tools, or inputs, it is essential to check the capability to look for any changes.
Context: In business, There are always tradeoffs between speed, consistency, and cost. For example, a highly capable process might be too slow or expensive.

You also need to compare apples to apples. Mixing data from multiple tools, dies, batches, operators, or equipment may fail to uncover root causes. For instance, your mean output may meet parameters, but you might miss opportunities to improve performance with a particular machine or die.

Are You Interested in Gaining More Quality Management Skills?

Suppose you are thinking about a career in quality management. In that case, you can jump-start your career by attending the IASSC-accredited program in Lean Six Sigma offered by Simplilearn in collaboration with the University of Massachusetts. This 24-week online Lean Six Sigma course offers live interactive classes using case studies and projects to solve real-world business problems and much more.

According to Glassdoor.com, Six Sigma Green Belt professionals can earn an annual average of $87,030 and go as high as $126,000. Check out the course, get the needed skills, and set your sights on a better career!

FAQs

Q: What is the definition of process capability?

A: Process capability is a metric that measures how well a process can produce output within specification limits.

Q: What is CP Cpk process capability?

A: Cp and Cpk are measures of process capability that compare the spread between the process specification limits and the process’s natural variability.

Q: What is a process capability example?

A: An example of process capability is measuring the variability in the weight of packaged cereal boxes to ensure the packaging process consistently produces boxes within the target weight range.

Q: What is the formula of process capability?

A: The formula for process capability Cp is:

(Upper Spec Limit – Lower Spec Limit) / (6 x Standard Deviation).

Q: Why do we use process capability?

A: We use process capability to quantify how well a process meets specifications, which helps determine if a process requires adjustment to improve consistency.

Q: What are the types of process capability?

A: The types are Cp (Process Capability), Cpk (Process Capability Index), or Pp (Preliminary Process Capability) and Ppk (Preliminary Process Capability Index).

Q: What is the Cpk formula?

A: The Cpk formula is: