Module 8. Statistical quality control
Lesson 26
BASIC CONCEPTS OF STATSITICAL QUALITY CONTROL
26.1 Introduction
From the early days of industrial production, the emphasis had been on turning out products of uniform quality by ensuring use of similar raw materials, identical machines, and proper training of the operators. Inspite of these efforts, the causes of irregularity often crept in inadvertently. Besides, the men and machines are not infallible and give rise to the variation in the quality of the product. For keeping this variation within limits, in earlier days, the method used was 100 per cent inspection at various stages of manufacturing.
It was in 1924 that Dr. W.A. Shewhart of Bell Telephone Laboratories, USA developed a method based on statistical principles for controlling quality of products during the manufacturing and thus eliminating the need for 100 per cent inspection. This technique which is meant to be an integral part of any production process, does not provide an automatic corrective action but acts as sensor and signal for the variation in the quality. Therefore, the effectiveness of this method depends on the promptness with which a necessary corrective action is carried out on the process. This technique has since been developed by adding to its armory more and more charts, as a result of its extensive use in the industry during and after the Second World War. In this lesson various terms used in the context of Statistical Quality Control (SQC) have been illustrated.
26.2 Definitions of Various Terms Involved in Statistical Quality Control
The following terms are used to understand the concept of Statistical Quality Control
26.2.1 Quality
The most important word in the term ‘Statistical Quality Control’ is quality. By ‘Quality’ we mean an attribute of the product that determines its fitness for use. Quality can be further defined as “Composite product characteristics of engineering and manufacture that determine the degree to which the product in use will meet the expectations of the customer at reasonable cost.” Quality means conformity with certain prescribed standards in terms of size, weight, strength, colour, taste, package etc.
26.2.2 Quality characteristics
Quality of a product (or service) depends upon the various characteristics that a product possesses. For example, the Kulfi we buy should have the following characteristics.
(a) TS (b) Sugar (c) Flavour (d) Body & Texture.
All these individual characteristics constitute the quality of Kulfi. Of course, some of them are important (critical) without which the Kulfi is not acceptable. For example Minimum TS, Sugar, Body and Texture score is important. However, other characteristics such as Colour and Flavour may not be so important. The quality characteristics may be defined as the “distinguishing” factor of the product in the appearance, performance, length of life, dependability, reliability, durability, maintainability, taste, colour, usefulness etc. Control of these quality characteristics in turn means the control of the quality of product.
26.2.3 Types of characteristics
There are two types of characteristics viz., variable characteristics and attribute characteristics.
26.2.3.1 Variable characteristic
Whenever a record is made of an actual measured quality characteristic, such as dimension expressed in mm, cm etc. quality is said to be expressed by variables. This type of quality characteristics includes e.g., dimension (length, height, thickness etc.),hardness, temperature, tensile strength, weight, moisture percent, yield percent, fat percent etc.
26.2.3.2 Attribute characteristic
Whenever a record shows only the number of articles conforming and the number of articles failing to conform to any specified requirements, it is said to be a record of data by ‘attributes’. These include:
· Things judged by visual examination
· Conformance judged by gauges
· Number of defects in a given surface area etc.
26.2.4 Control
Control means organizing the following steps:
· Setting up standards of performance.
· Comparing the actual observations against the standards.
· Taking corrective action whenever necessary.
· Modifying the standards if necessary.
26.2.5 Quality control
Quality control is a powerful productivity technique for effective diagnosis of lack of quality (or conformity to set standards) in any of the materials, processes, machines or end products. It is essential that the end products possess the qualities that the consumer expects of them, for the progress of the industry depends on the successful marketing of products. Quality control ensures this by insisting on quality specifications all along the line from the arrival of materials through each of their processing to the final delivery of goods.Quality control, therefore, covers all the factors and processes of production which may be broadly classified as follows:
· Quality of materials: Material of good quality will result in smooth processing there by reducing the waste and increasing the output. It will also give better finish to end products.
· Quality of manpower: Trained and qualified personnel will give increased efficiency due to the better quality production through the application of skill and also reduce production cost and waste.
· Quality of machines: Better quality equipment will result in efficient working due to lack or scarcity of break downs thus reducing the cost of defectives.
· Quality of Management: A good management is imperative for increase in efficiency, harmony in relations, growth of business and markets.
Variation in the quality of the manufactured product in the repetitive process in the industry is inherent and inevitable. These variations are broadly classified as being due to two causes viz., (i) chance causes, and (ii) assignable causes.
26.2.6.1 Chance causes
Some “Stable pattern of variation” or “a constant cause system” is inherent in any particular scheme of production and inspection. This pattern results from many minor causes that behave in a random manner. The variation due to these causes is beyond the control of human being and cannot be prevented or eliminated under any circumstance. Such type of variation has got to be allowed within the stable pattern, usually termed as Allowable Variation. The range of such variation is known as natural tolerance of the process.
26.2.6.2 Assignable causes
The second type of variation attributed to any production process is due to non-random or the so called assignable causes and is termed as Preventable Variation. The assignable causes may creep in at any stage of the process, right from the arrival of raw materials to the final delivery of the goods.
Some of the important factors of assignable causes of variation are substandard or defective raw material, new techniques or operations, negligence of the operators, wrong or improper handling of machines, faulty equipment, unskilled or inexperienced technical staff and so on. These causes can be identified and eliminated and are to be discovered in a production process before it goes wrong i.e., before the production becomes defective.
26.3 Statistical Quality Control
By Statistical Quality Control (SQC) we mean the various statistical methods used for the maintenance of quality in a continuous flow of manufactured goods. The main purpose of SQC is to devise statistical techniques which help us in separating the assignable causes from chance causes of variation thus enabling us to take remedial action wherever assignable causes are present. The elimination of assignable causes of erratic fluctuations is described as bringing a process under control. A production process is said to be in a state of statistical control if it is governed by chance causes alone, in the absence of assignable causes of variation.
In the above problem, the main aim is to control the manufacturing process so that the proportion of defective items is not excessively large. This is known as ‘Process Control’. In another type of problem we want to ensure that lots of manufactured goods do not contain an excessively large proportion of defective items. This is known as ‘Product or Lot Control’. The process control and product control are two distinct problems, because even when the process is in control, so that the proportion of defective products for the entire output over a long period will not be large, an individual lot of items may not be of satisfactory quality. Process Control is achieved mainly through the technique of ‘Control Charts’ whereas Product Control is achieved through ‘Sampling Inspection’.
26.4 Stages of Production Process
Before production starts, a decision is necessary as to what is to be made. Next comes the actual manufacturing of the product. Finally it must be determined whether the product manufactured is what was intended. It is therefore necessary that quality of manufactured product may be looked at in terms of three functions of specification, production and inspection.
26.4.1 Specification
This tells us what is to be produced and of what specification. That is, it gives us dimension and limits within which dimension can vary. These specifications are laid down by the manufacturer.
26.4.2 Production
Here we should look into what we have manufactured and what was intended to.
26.4.3 Inspection
Here we examine with the help of SQC techniques whether the manufactured goods are within the specified limits or whether there is any necessity to widen the specifications or not. So SQC tells us as to what are the capabilities of the production process.
Therefore statistical quality control is considered as a kit of tools, which may influence decisions, related to the functions of specification, production or inspection. The effective use of SQC generally requires cooperation among those responsible for these three different functions or decisions at a higher level than any one of them. For this reason, the techniques should be understood at a management level that encompasses all the three functions.