Introduction:
A natural key output variable (KOV) is the amount of money or value of goods stolen. Since the amount is a single continuous variable, Xbar & R charting is the most relevant of the methods. The usual inventory counts that are likely in place can be viewed as complete inspection with regard to property theft.
Measuring Quality Using SPC Charting:
1. Statistical process control (SPC) charting methods aid in the efficient monitoring of systems. If applied as intended, they increase the chance that skilled employees will only be tasked in ways that reward their efforts.
2. The section begins with a discussion of the concepts invented by Shewhart in the 1920s and refined years later by Deming and others.
3. Four widely used charting procedures are described:
4. P-charting,
a) U-charting,
b) Demerit charting,
c) Xbar & R charting
5. Each method generates a measurement of the relevant process quality. These measures provide a benchmark for any later project phases to improve.
Concepts: Common Causes and Assignable Causes:
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In 1931, Shewhart formally proposed the Xbar & R charting method he invented while working at Bell Telephone Laboratories (see the re-published version in Shewhart 1980).
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Shewhart had been influenced by the mass production system that Henry Ford helped to create.
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In mass production, a small number of skilled laborers were mixed with thousands of other workers on a large number of assembly lines producing millions of products.
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Even with the advent of Toyota’s lean production in the second half of the twentieth century and the increase of service sector jobs such as education, health care, and retail, many of the problems addressed by Shewhart’s method are relevant in today’s workplace.
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Sometimes variation causes characteristic values go outside the specification limits, and units become nonconforming. Typically, most of the units conform to specifications. Therefore, skilled labor generally views variation as an “evil” or negative issue. Without it, one hundred percent of units would conform.
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The phrase “common cause variation” refers to changes in the system outputs or quality characteristic values under usual circumstances. The phrase “local authority” refers to the people (not shown) working on the production lines and local skilled labor. Most of the variation in the characteristic numbers occurs because of the changing of factors that local authority cannot control.
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If the people and systems could control the factors and make all the quality characteristics constant, they would do so. Attempts to control the factors that produce common cause variation generally waste time and add variation.
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The term “over-control” refers to a foolish attempt to dampen common cause variation that actually increases it. Only a large, management-supported improvement project can reduce the magnitude of common cause variation.
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The phrase “assignable cause” refers to a change in the system inputs that can be reset or resolved by local authority.
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Evaluation of the magnitude of the common cause variation, providing a benchmark for quality improvement or design activities;
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Monitoring and identification of assignable causes to alert local authority in a timely manner (something might be fixable); and Discouraging local authority from meddling unless assignable causes are identified.
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The third goal follows because local authority’s efforts to reduce common cause variation are generally counterproductive.
Example (Theft in Retailing) A retail executive is interested in benchmarking theft at five outlets prior to the implementation of new corporate anti-theft policies. List one possible source of common cause variation and assignable cause variation.
Answer: Lone customers and employees stealing small items from the floor or warehouse contribute to common cause variation. A conspiracy of multiple employees systematically stealing might be terminated by local management.
