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Process Capability Improvement : After all special causes have been corrected and the process is running

in statistical control, the process capability can be assessed. If the variation from common causes is

excessive, the process cannot produce output that consistently meets customer needs. The process

itself must be investigated, and management action must be taken to improve system.

For Continuing process improvement, repeat these three phases. Gather more data as appropriate;work to reduce process variation by operating the process in statistical control and continually

improving its capability.

ACTIONS taken to combat the above types of variation include,

Local actions are those that can be taken by people closed to the process and are usuallyrequired for eliminating special causes of variation. These actions can only correct about 15% or

the process problems.

Actions on the system are those that almost always require management action for correctionand are usually required for eliminating common causes of variation. These action can correct

about 85% of the process problem.

Common Causes generally account for about 80 to 90 percent of the observed variation in a production

process. The remaining 10 to 20 percent result from special causes of variation, often called assignable

causes.

Special causes arise from external sources that are not inherent in the process. A bad batch or material

purchased from a supplier, poorly trained operator, excessive tools wear, or miss-calibration of

measuring instruments are example of special causes.

Special causes result in unnatural variations that disrupt the random pattern of common causes.

Hence they are generally easy to detect using statistical methods, and it is usually economical to remove

them.

A system governed only by common causes is said to be stable. Understanding a stable system and the

differences between special and common causes of variation is essential for managing nay system.

Management can make two fundamental mistakes in attempting to improve a process:

1. To treat as special cause any fault, complaint, mistake, breakdown, accident, or shortage whenit actually came from common causes.

2. To attribute to common causes any fault, complaint, mistake, breakdown, accident, or shortagewhen it actually came from a special cause.

In the first case, tampering with a stable system will actually increase the variation in the system. In thesecond case, we can miss the opportunity to eliminate unwanted variation by assuming that it is not

controllable. Changing a system on the basis of a special cause can damage the system and add cost.

Variation should be minimized. The producer and consumer both benefit from reduced variation. The

producer benefits by having less need for inspection, less scrap and rework, and higher productivity. The

consumer is assured that all products have similar quality characteristics; this especially important when

the consumer is another firm using large quantities of the product in its own manufacturing or service

operation.

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Variation increases the cost of doing business. The only way to reduce variation due to common causes

is to change the technology of the process -the machines, people, materials, methods, or measurement

system. The process is under the control of management, not the production operators. Pressuring

operators to perform at higher quality levels may not be possible and may be counterproductive.

Variation due to special causes can be identified through the use of control charts, which shall be

introduced latter. (Page No. 54, Book : 32876995-TOTAL-QUALITY-MANAGEMENT-NOTES.PDF)