Variance

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Variation Control

Variation is a basic phenomenon of nature. This effects all entities including products and processes. Variation is found in all stages of product life cycle including design & development, manufacturing, service and supplier processes. Controlling process variation is a key to achieving Six Sigma quality.

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Understanding & controlling variation

Variation is responsible for the difference between one unit of product and another. It can also be defined as the difference between specifications and customer requirements. Variation is present in all processes. When it is present in one or more characteristics of a product or process, it causes poor quality and customer dissatisfaction.

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Understanding & controlling variation

Products and processes are expected to vary because no two things are exactly alike. Differences result from material characteristics, methods, people, machine and environmental factors as depicted on the next slide.

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Sources of variation

Material

•Assemblies •Components •Suppliers •Consumables

Methods

•Procedures •Policies •Accounting

Environment

•Noise level •Humidity •Temperature •Lighting

Variation

People

•Training •Experience •Skill •Attitude

Machine

•Technology •Variability •Tooling •Fixtures

Measurement

•Counting •Instruments •Gauging •Tests

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Causes of variation

Causes

Chance / Inherent Causes They have the influence on the output all the time.

Special / Assignable Causes They influence the output only once in a while.

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Chance / inherent causes of variation

Chance or inherent causes are sources of variation which are always present because of small day-to-day variables. These causes are inherently part of the process (or system) and affect everyone working in the process. They are typically due to a large number of small random sources of variation. Chance causes also contribute...