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Statistical

Quality

Control

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What is SQC ?

Statistical quality control (SQC) is the term used to describe the set of statistical tools used by quality professionals.

The application of statistical techniques to measure and evaluate the quality of a product, service, or process.

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History

SQC was pioneered by Walter A. Shewhart at Bell Laboratories in the early 1920s.

Shewhart developed the control chart in 1924 and the concept of a state of statistical control.

Shewhart consulted with Colonel Leslie E. Simon in the application of control charts to munitions manufacture at the Army in 1934.  

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History

 W. Edwards Deming invited Shewhart to speak at the Graduate School of the U.S. Department of Agriculture, and served as the editor of Shewhart's book  Statistical Method from the Viewpoint of Quality Control (1939) which was the result of that lecture.

Deming was an important architect of the quality control short courses

that trained American industry in the new techniques during WWII. 

Deming traveled to Japan during the Allied Occupation and met

with the Union of Japanese Scientists and Engineers(JUSE)in an

effort to introduce SQC methods to Japanese industry

5Characteristics of SQC

It is designed to control the quality standard of goods produced for marketing.

It is exercise by the producers during the production process

It is carried out with the help of certain statistical tools.

It is designed to determine the variations in quality of the goods .

It aims to ascertain whether the production process is in control or not, and whether the products are of specified quality.

It is an economical measure of assessing the quality standard of goods through statistical experiments without checking every product in detail.

6Advantages of SQC

It provides a means of detecting error at inspection.

It revels whether the production process is in control or not.

It leads to more uniform quality of production.

It improves the relationship with the customer, reduced customer complaints

Reduction of Scrap.

It reduces the number of rejects and saves the cost of material.

It reduces inspection costs.

It leads to more uniform quality of product

7Limitations of SQC

It cannot be applies indiscriminately as a solution to all the quality evils

It leads to a false sense of security in the absence of general quality awareness.

It provides only an information service, and it can not reduce the managers responsibility,

It cannot be applied mechanically to all production process without studying their peculiar environments.

It involves mathematical and statistical problems in the process of analysis.

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SQC Categories

Descriptive statistics

Statistical process control (SPC)

Acceptance sampling

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Descriptive Statistics

Descriptive statistics are used to describe quality characteristics and relationships.

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Descriptive Statistics

The Mean- measure of central tendency

The Range- difference between largest/smallest observations in a set of data

Standard Deviation measures the amount of data dispersion around mean

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Statistical Process Control

Statistical process control (SPC) involves inspecting a random sample of the output from a process and deciding whether the process is producing products with characteristics that fall within a predetermined range. SPC answers the question of whether the process is functioning properly or not

Extend the use of descriptive statistics to monitor the quality of the product and process

Statistical process control help to determine the amount of variation

To make sure the process is in a state of control

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Acceptance sampling

Acceptance sampling is an important field of statistical

quality control that was popularized by Dodge and Romig

and originally applied by the U.S. military to the testing of

bullets during World War II.

Acceptance sampling is the process of randomly inspecting

a sample of goods and deciding whether to accept the

entire lot based on the results. Acceptance sampling

determines whether a batch of goods should be accepted

or rejected

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Variation in Quality

No two items are exactly alike.

Some sort of variations in the two items is bound to be there. In fact it is an integral part of any manufacturing process.

This difference in characteristics known as variation.

This variation may be due to substandard quality of raw material, carelessness on the part of operator, fault in machinery system etc..

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Types Of Variations

Variation due to “CHANCE CAUSES”

Variation due to “ASSIGNABLE CAUSES”

Variation due to chance causes/common causes

Variation occurred due to chance.

This variation is NOT due to defect in machine, Raw material or any other factors.

Behave in “random manner”.

Negligible but Inevitable

The process is said to be under the state of statistical control.

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Variation due to assignable causes

Non – random causes like: Difference in quality of raw material

Difference in machines

Difference in operators

Difference of time

It should be identified and corrected.

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Thank You…

By ;Suresh T SM.Com 314