Statistical Quality Control (S.Q.C.) : Meaning, Purpose, Quality of a product, Chance and Assignable Variation, Process Control and Product Control.

 

Statistical Quality Control (S.Q.C.) ©

 

Quality:

In consumer's view, quality of a product means fitness of product for use, while in manufacturer's view, quality means conformance to specification.

Def":

Quality is one or more characteristics that a product should possess.

Dimensions of quality:

1. Performance- Will the product do the intended job?

2. Reliability- How often does the product fail?

3. Durability- How long does the product last?

4. Serviceability- How easy is it to repair the product?

5. Aesthetics - How does the product look like?

6. Features - What does the product do?

7. Perceived Quality- What is the reputation of the company or its product?

8. Conformance to standards - Is the product made exactly as the designer intended?

 

Quality Characteristics:

The factors that jointly describe the quality of the product are-

i)  Physical               - Length, Weight, Voltage

ii) Sensory               - Taste, Appearance, Color

iii) Time oriented    - Reliability, Durability, and Serviceability

 

S.Q.C.:

Meaning-

By statistical quality control, we mean the various statistical methods, used for the maintenance of quality in a manufacturing product. In every manufacturing process, it is essential that product should possess the quality which the consumer wants. But in any manufacturing process, it is not possible to produce products of exactly the same quality. For every process, variation in the quality of manufactured products is inevitable.

This variation may be due to two causes: 1) Chance causes

                                                                  2) Assignable causes

 

Purpose:

The main purpose of S.Q.C. is to device the statistical methods, for separating chance causes from assignable causes; so that we may take appropriate steps as quickly as possible whenever assignable causes are present in the process Our aim is to control the manufacturing process so that the proportion of items is not excessively large.

 

Chance causes:

In any manufacturing process & inspection, some stable pattern of variation is inherent. This pattern of variation is due to many minor causes, which behaves in random manner. This causes cannot be prevented, therefore this variation is called allowable. For ex: there may be a slight variation in the electric current supplied to the machine, there may be slight vibrations of the machines. The variation in the quality due to such factors is very small & tolerable.

 

Assignable causes:

The second type of variation in any production process is due to non random causes & called as assignable causes. The assignable causes includes the factors like raw material of bad quality, improper setting of machines, wrong handling of machines, machine defects, unskilled workers etc. Variations caused by these factors are generally high & cannot be tolerable. When such causes are present, quality of the product is lost. Assignable causes can be identified & eliminated from the process by taking proper care.

Process control:

In any manufacturing process, the quality of a product is affected by chance & assignable causes. If the items produced by the production process indicate the presence of assignable causes we may search for assignable causes & remove them from the production process, so that the items produced by the process will be free from assignable causes & hence of good quality. This is known as 'process control' & is achieved by the technique of control charts.

Product control (Lot control) :

Product control is concerned with the inspection of items already produced. After inspection of items it is decided whether to accept the lot or not. Product control is mainly achieved by ‘acceptance sampling plans’.

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