What is Process Capability and Overview

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Dec 02, 2022

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What is Process Capability?

Process capability analysis is one of the most important tools used in Six Sigma. It is based on the assumption that if a process has high variation, then it will be difficult to maintain consistent quality. The main goal of process capability analysis is to determine whether or not a process is capable enough to meet customer requirements in terms of quality and cost.

Process capability is often used as input into statistical process control (SPC) charts, which are used to monitor and improve processes.

A typical use case for process capability analysis is when you're working with suppliers on improving their quality and want to understand how well they are performing relative to your expectations.

Process Tolerance

A process that has wide tolerances is said to have poor process capability. Such a process will produce large amounts of defective products, resulting in higher costs. A process with narrow tolerances can produce only limited quantities, resulting in less market share.

Overview of Process Capability

Process capability is a measure of the ability of a process to produce output within specified limits. Process capability is normally expressed as a number between 1 and -1, where zero indicates that the process is incapable of producing an output within the prescribed limits. Higher values indicate greater consistency and repeatability in the output of a given process.

Process capability analysis determines whether or not a process can be improved by adjusting production parameters (such as machine settings and raw materials). This analysis also allows manufacturers to compare their processes against those used by other companies to assess their relative quality levels.

Process capability analysis focuses on three key measures:

1. Process mean (average) deviation from target: This is the average difference between the actual and target values for a measured characteristic (such as weight, volume or time). The greater this deviation, the lower the process capability.

2. Process standard deviation: This is a measure of variation in performance around the mean value. The smaller it is, the more consistent are measurements of a given characteristic across time or across samples taken at different times.

3. Process capability index: This provides an indication of how close actual results are to target results (zero means perfect conformance with target values).

Need for Process Capability

Here are some of the cases when process capability is needed:

1. Compare processes and products across different industries, companies and locations

Process capability allows you to compare your processes with other similar processes across different industries, companies and locations. For instance, if you have an automotive manufacturing process, and you want to compare it with another automotive manufacturing process in another country or region, process capability can help you do that effectively.

2. Compare processes in different locations and organizations within the same company

If there are two assembly lines producing product A and product B, respectively, but both products require different tolerances, then you can still compare them using process capability.

3. Determine if a process is capable enough to meet customer requirements in terms of quality and cost

Process capability helps you determine whether or not your product meets customer specifications. This can be done by comparing measurements taken on samples against customer requirements or by comparing measurements taken at different times within the same batch. This will help determine whether or not any problems have affected the quality of products being produced and provide insight into what may need improvement.

4. Achieve zero defects

The goal of most businesses is to achieve zero defects in their products. However, this is easier said than done since there are many factors that can affect quality such as human error, equipment failure and other external factors out of the control of management such as weather conditions and natural disasters affecting raw material supplies

5. Monitor ongoing improvements as part of your continuous improvement plan

One of the most important aspects of process capability is its ability to help you monitor ongoing improvements. If you're constantly making changes to your processes, you're going to want something that can help you understand how effective these changes are. Process capability allows you to measure the effectiveness of these changes so they can be improved upon if necessary.

Factors Influencing Process Capability

Process capability is measured by calculating the upper and lower specification limits, which are then compared to the actual results. Five factors determine the capability of a process:

1. Process variation

Process variation is the difference between the actual output of a process and its target value. A process that generates outputs that are all within a certain range around the target value has low process variation. Processes with high process variation produce outputs that vary significantly from the target value.

Processes with high process variation are less predictable, more costly, and more likely to result in defective products than those with low process variation. Methods with low process variation are easier to control and produce high-quality products consistently.

2. Limits of tolerance

This is the maximum amount of variation that is allowed in a process before it is considered acceptable for use. A low limit of tolerance means that there is less room for error or variation in the process, which means that you need to keep it as consistent as possible. If a product has a low limit of tolerance, then you may need extra resources and workers to keep everything running smoothly.

On the other hand, if your process has high limits of tolerance, then there is more room for error or variation in the process than necessary. This can result in wasted resources and time trying to correct problems instead of improving your product quality.

3. Sampling plan

The sampling plan is the process used to select a sample from the entire population. It is important to use a method that is as unbiased as possible. If the process has a lot of variabilities, you will need to take more samples to make sure that you have enough information about the process. If your process has little variation, it will be easier to determine its capability using fewer samples.

4. Statistical tools (mean and standard deviation)

Statistical tools are the key to process capability. They can help you determine whether a process is in control, how well the process is performing and whether changes to the process have improved it.

These are used to calculate process capability indices and check for their suitability for use with a particular application.

5. Measurement system accuracy

Measurement systems are prone to error because they are made up of multiple components that interact with each other and the environment. For example, if your measurement system has a 0% accuracy rate, then all measurements will be off by 100%. This can have severe implications on your ability to consistently meet specifications.

Process Capability Indices (Cp and Cpk)

Process capability indices help you determine if a process is capable of meeting customer requirements. The indices are calculated from the same data that are used to calculate the control limits, but with different formulas.

The most commonly used indices are Cp and Cpk. Both these indices can be calculated from either the mean or range charts.

Cp is the upper specification limit divided by the standard deviation of the process output. Cp can be expressed as a decimal, per cent or fraction.

Cp = USL - LSL/ 6σ


where USL stands for Upper Specification Limit and LSL stands for Lower Specification Limit.

A high Cp value indicates that the process is capable of producing high-quality products, whereas a low Cp value indicates that there is likely to be significant variation in product characteristics which could potentially lead to poor quality.

The Cp index can also be used to determine if a process will be able to meet customer requirements in the long term by evaluating changes in the underlying process parameters over time (e.g., raw material and equipment variability).

Cpk is the product of the upper specification limit and 3 Sigma divided by 6 Sigma. It is expressed as a decimal or per cent.

Cpk = Cp (1-k)

Here, k is the distance between the midpoint of the specification range and the process mean.

If Cpk is greater than 1, then the process is running more efficiently than planned. If Cpk is less than 1, then there are problems with the process and it needs improvement.

Assessment of Process Capability

The assessment of process capability is a measure of the quality and consistency of a process. It is a measure of the ability of a process to produce output within the specification limits. The output can be either a physical product or service, and it can be quantified by either count, time, money or any other metric.

Wrapping Up

Process capability is an important aspect of Lean Six Sigma. It's a way of measuring the performance of a process.

It's an important concept because even if a process is well-designed and well-implemented, it needs to be capable of meeting customer expectations.

You can get started with process capability with Lean Six Sigma Certification online courses.

The Lean Six Sigma Certification online helps you understand how to use data effectively to improve your processes. It teaches you how to get the most out of your current resources and make sure that they are working at their full capacity.


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