Using Quality Analyst to produce Cpk and DPMO reports

Do you need to provide customers with CpK (Process Capability) and DPMO (Defects Per Million Opportunities) reports? Does the paperwork constantly bog you down?

Are you going for ISO, TS16949 and AS9000 accreditation, and need to show evidence of a Quality Control Procedure?

Are you looking to reduce variation, and need to identify the most cost effective improvements to make?

If the answer to any of these questions is yes, then you need a customised SPC solution. NWA Quality Analyst allows you to automatically produce Cpk and DPMO reports to effectively analyse the quality of your data, as well as comply with ISO9000, QS9000 and TS16949 standards.

Cpk Reporting

The rapid pace of change in highly competitive industrial environments requires speedy and efficient decisions and actions, and an automated statistical method which minimises sampling during these studies hands any organisation which uses them a great advantage - Cpk reporting can result in 50% sampling savings when used regularly with other forms of analysis and charting.

Cpk reporting is used for Variable Data, which is quanititative and can be divided into (Discrete) count data and (Continuous) data. Examples are:

• Length
• Weight
• Volume
• Time to complete
• etc.

Studies are usually performed whenever:

• The capability of a process to meet customer specifications needs to be determined
• engineering tolerances are reviewed against the observed variability of the process and/or new equipment is evaluated
• process changes and/or improvements need to be evaluated.

A definition of Cpk:

The process potential index, or Cp, measures a process's potential capability, which is defined as the allowable spread over the actual spread. The allowable spread is the difference between the upper specification limit and the lower specification limit. The actual spread is determined from the process data collected and is calculated by multiplying six times the standard deviation, s. The standard deviation quantifies a process's variability. As the standard deviation increases in a process, the Cp decreases in value. As the standard deviation decreases (i.e., as the process becomes less variable), the Cp increases in value.

By convention, when a process has a Cp value less than 1.0, it is considered potentially incapable of meeting specification requirements. Conversely, when a process Cp is greater than or equal to 1.0, the process has the potential of being capable.

Ideally, the Cp should be as high as possible. The higher the Cp, the lower the variability with respect to the specification limits. In a process qualified as a Six Sigma process (i.e., one that allows plus or minus six standard deviations within the specifications limits), the Cp is greater than or equal to 2.0. However, a high Cp value doesn't guarantee a production process falls within specification limits because the Cp value doesn't imply that the actual spread coincides with the allowable spread (i.e., the specification limits). This is why the Cp is called the process potential.

The process capability index, or Cpk, measures a process's ability to create product within specification limits. Cpk represents the difference between the actual process average and the closest specification limit over the standard deviation, times three.

By convention, when the Cpk is less than one, the process is referred to as incapable. When the Cpk is greater than or equal to one, the process is considered capable of producing a product within specification limits. In a Six Sigma process, the Cpk equals 2.0.

The Cpk is inversely proportional to the standard deviation, or variability, of a process. The higher the Cpk, the narrower the process distribution as compared with the specification limits, and the more uniform the product. As the standard deviation increases, the Cpk index decreases. At the same time, the potential to create product outside the specification limits increases.

Cpk can only have positive values. It will equal zero when the actual process average matches or falls outside one of the specification limits. The Cpk index can never be greater than the Cp, only equal to it. This happens when the actual process average falls in the middle of the specification limits.

DPMO Reporting

From DPMO data, you can identify and prioritise assembly process improvements. DPMO is an invaluable reference for enhancing the design for manufacturing (DFM) process and predicting yield at design stage.

DPMO is used for Attribute Data, which is purely binary in nature. It is for:

• Defects
• Rejects

DPMO reporting takes place when checking your final product. It factors in product complexity and allows comparing assembly performances for different designs, different processes and different types of assembly environments. DPMO is the reference of choice for measuring and comparing process quality achievements because it is a robust and portable metric.

A definition of DPMO:

DPMO is the total number of defects divided by the total number of opportunities for a defect, multiplied by 1,000,000. To operate at Six Sigma level means a process operates at not more than 3.4 defects per million opportunities.

Defects are any critical characteristic of a sampled entity that fails to meet customer expectations.

Opportunities are critical characteristics of a sampled entity that could either meet or fail customer expectation. In many manufacturing situations there may be multiple opportunities per entity, whereas transactional situations often either meet the customer's requirements or they don't. When evaluating potential opportunities (and defects) it is important to work at the customer-significant functional level. Defect opportunities must be critical  to the customer, be independent of each other, and only increase numerically with increased complexity. For example, in a book, defects could be misspelled words, grammatically incorrect sentences, missing text or pages and so on - however, readers may regard the 'sentence' as the functional entity of interest, and a defect is a spelling or grammatical error in each sentence.

For more information on improving yields with Statistical Process Control, click here. This link acts as a gateway to the NWA website, an external site.

If you have any further questions about using Quality Analyst to produce Cpk and DPMO reports, please contact our Quality Analyst team.

To download a free 30-day Quality Analyst demo, click here.

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