SPC

Statistical Methods used in Quality Improvement

SPC and Capability Analysis

Acceptance Sampling

Definition of Quality

Traditional's View : Fitness for use

Modern' s View: Proportional to variability

Variation

Natural Variation Unnatural Variation

Chance Causes Assignable Causes

Common Causes Special Causes

Chronic Problems Sporadic Problems

Description

NV consists of many individual causes

UV consists of just one or just a few individual causes

Any one chance cause results in a minute amount of variation

Any one assignable cause can result in a large amount of variation

Example:
Man
Machines
Materials
Example:
Man
Machines
Materials

Interpretation

Chance cause cannot economical eliminated from a process

Assignable causes can be detected and action to eliminate them is usually economically justified

With only natural variation, the process is in control and predictable

With unnatural variation present, the process is not stable and unpredictable

An observation within the control limits means that the process is in control and shall not be adjusted

An observation beyond the control limits means that the process shall be investigated and corrected

Further or Necessary Action

Reduction of NV requires Management intervention at the system level by making changes to the system

Elimination of UV requires Operator intervention at the Operation level by fixing local faults

Process Capability Ratio C_p, is a measure of the ability to manufacture product that meets the specification.

            Specification Limit        Percentage Inside specification        ppm Defective

            +/- 1 sigma                    68.27                                             317300
            +/- 2 sigma                    95.45                                             45500
            +/- 3 sigma                    99.73                                             2700
            +/- 4 sigma                    99.9937                                          63
            +/- 5 sigma                    99.999943                                       0.57
            +/- 6 sigma                    99.9999998                                     0.002

            For a +/- 3 sigma control limits, there will be a 0.27% chance the process points will fall out of
            the limits when the process is in control. In another words, for every 370 (1/0.27%) points,
            there will be an out of control point.

	Statistical Methods used in Quality Improvement
	SPC and Capability Analysis
	Acceptance Sampling

Variation
Natural Variation	Unnatural Variation
Chance Causes	Assignable Causes
Common Causes	Special Causes
Chronic Problems	Sporadic Problems
Description
NV consists of many individual causes	UV consists of just one or just a few individual causes
Any one chance cause results in a minute amount of variation	Any one assignable cause can result in a large amount of variation
Example: Man Machines Materials	Example: Man Machines Materials
Interpretation
Chance cause cannot economical eliminated from a process	Assignable causes can be detected and action to eliminate them is usually economically justified
With only natural variation, the process is in control and predictable	With unnatural variation present, the process is not stable and unpredictable
An observation within the control limits means that the process is in control and shall not be adjusted	An observation beyond the control limits means that the process shall be investigated and corrected
Further or Necessary Action
Reduction of NV requires Management intervention at the system level by making changes to the system	Elimination of UV requires Operator intervention at the Operation level by fixing local faults