Capability measures how well the process of a product is able to stay within the product’s determined specifications over the long term. Manufacturing capability in particular describes what a company is able to produce. It also establishes approved and standardized procedures for achieving that capability or otherwise producing a specific result.

Achieving capability is a standard that many companies aim for, as it lends to stability. Once a manufacturing process becomes capable, this indicates that its output (the product) is predictable. Not only does this streamline the production process and reduce waste, but it also helps make accurate predictions for the future in terms of costs and returns.

A process is considered to be capable—and stable—if it is influenced only by common causes of variation. These common causes are naturally occurring within the production system, and not only affect all workers but also all outcomes. Examples of common variations include normal wear and tear, poor working conditions, and computer response times.

There are several indices that may be used to calculate capability. A popular capability index is a measure known as the process capability measure (Cpk). It estimates the capability of a stable process. In this, common causes of variation are included, but unusual or one-off causes are not. Cpk also indicates how well-centered the particular process is within specification limits. The value of Cpk varies from 1 to 2:

  • Cpk= 1.00, process is not capable
  • 1< Cpk < 1.33, process capability is poor
  • 1.33< Cpk < 1.67, process capability is moderate
  • 1.67< Cpk < 2, process is well capable
  • Cpk= 2, process is fully capable

The greater the process capability, the lower the rate of actual defects in the product, lending to stability. When considering process capability, it’s important to have an extensive understanding of the variations within your manufacturing process. From there, you can work to reduce these variations.

When a process is thoroughly capable and has a Cpk value of 2, this lends to a Six Sigma process. In fact, capability is an important component of Six Sigma, a process improvement methodology that identifies defects and wastes, then implements ways to resolve them. If this methodology is conducted correctly, all manufacturing processes will have minimal variability—which achieves exactly what capability also aims to achieve: stability. 

A capable process generates uniformity in a manufacturer’s product output. This results in a variety of benefits, including:

  • Low defect rates
  • Positive financial implications
  • Quality improvement
  • Products align closer to customer requirements, or closer to the needs of the business
  • Lowers the levels of waste, repair, and rework 

Capability, and Cpk, can be applied usefully in a variety of work environments. A key advantage to the process capability measure is that it’s simple to determine, yet if properly utilized, is a strong influence in driving improvement within a manufacturing process. If you need to implement more stability within your manufacturing processes and would like to incorporate Six Sigma, understanding the concept of capability and its impact is an essential first step. Over time, it leads to process and product improvement.


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