Introduction to Statistical Process Control. Muhammad Amir Aslam
current status quo and any deviation of the prevailing policies of the organization.
The important concern of the SPC is based on the use of sampling techniques. The already developed methodologies are the single, double, with plenty of literature available in the books of SPC, but the repetitive sampling and multiple dependent state sampling schemes, which are the most commonly used in quite near past, are explained in this book. Many other sampling schemes, which are also tenderly accepted in SPC, are the probability to ratio sampling scheme, the ranked set sampling, and fast initial response set sampling scheme.
1.2 History and Background of Control Charts
Most of the SPC techniques being used nowadays are the techniques developed during the twentieth century. The control chart technique was basically introduced by Walter A. Shewhart in 1920s during his services rendered to Bell Laboratories. Two types of variations (common and special causes of variations) in the products of the manufacturing units were pointed out in an internal memo of about one page length written by Walter A. Shewhart on 16 May 1924. That one page consisted of text, and one‐third of the page was utilized for the diagram of the shape showing UCL and LCL, which we are using nowadays as control chart. Two renowned quality control experts, connoisseurs, aficionadas, buffs, and polishers of the statistical methods applied to the quality management are Shewhart and Deming who wrote a book, Statistical Methods from the Viewpoint of Quality Control, in 1939, which is helpful and supportive even today as it was then (Oakland, 2008, p. 14). After the defeat of Japan in Second World War, Dr. William Edwards Deming (Shewhart's boss), basically an American statistician, engineer, professor, author, and management consultant, helped Japanese companies to improve the quality of the products by focusing on the monitoring and diagnosing of variability. The causes of variation were focused particularly in the manufacturing industry to improve performance through quality management system and SPC. Many companies in the world adopted the Deming philosophy and attained success swiftly in the years to come. The popularity of SPC for the success of manufacturing industry delivered the message to suppliers of raw material, goods and services providing organizations, and all the companies related to the manufacturing industry about the enormous potentials of the SPC in terms of market share, maximizing profits, and reducing rework of products. As a result a colossal demand for quality control techniques, SPC experts, and utilization of computer technology was created to survive in the world.
In 1935 the British Standards Institution introduced modification as the three‐sigma control limits were replaced with the limits based on the percentiles of normal distribution.
Since the early 1980s, the US industry improved the quality substantially due to theories developed by the researchers. Dr. Genichi Taguchi, Dr. Joseph M. Juran, Dr. Deming, and Philip Crosby assisted the manufacturing industry magnificently for improving the product quality. A new boast was injected to the industry by Dr. Genichi Taguchi by introducing new concepts in experimental design, robust design, and loss function. The quality system of ISO 9000 and QS 9000 was introduced by the United States in the early 1990s.
Most of the emphasis on the execution of the control chart has been seen in the second half of the twentieth century when the monitoring of small shifts in the production process was focused by announcing two methodologies. These methodologies were the cumulative sum introduced by E. S. Page in 1954 and the EWMA by S. W. Roberts in 1965. Another direction was introduced by R. E. Sherman in 1965 when he hovered the idea of repetitive group sampling. The technique of the repetitive sampling was readdressed by S. Balamurali and C. H. Jun in 2006. Currently, a plenty of literature can be seen on the application of repetitive group sampling technique with the major contributions of M. Aslam and L. Ahmad (Ahmad, Aslam, & Jun, 2014; Aslam, Srinivasa Rao, Ahmad, & Jun, 2017).
1.3 What Is Quality and Quality Improvement?
Quality may be defined as the characteristic of a good or service that can fulfill our desire. In other sense, it is defined as the fitness to use. This quality of product is used as a powerful tool for deciding/selecting from many competing products. This quality plays a vital role in the growth, business success, and to face the competing market. It is a widespread phenomenon as the quality may be an individual, a departmental store, a food chain, a cellular company, a transport company, etc. The quality may be classified as the quality of the design and the quality of the conformance. The quality of design may be described as the goods and the services produced in various designs and levels. The quality of the product like other corporate matters must be reviewed continually in the light of current settings. Two of the most famous authors, Shewhart and Deming, worked for the quality and management of the quality using statistical theories and concepts.
The quality of conformance depends on several factors, such as specifications of the design, selection of manufacturing process, types of process control, etc. Garvin (1987) described eight dimensions of quality. These dimensions are explained as follows:
1 Performance
How much the product is worth for the intended job? The product or services are evaluated for its specific functions. If the desired functions are performed well, then the performance of the product is high.
1 Reliability
How often does the product fail to perform? When the use of the product is fit for a long time, then it is said the product is reliable for a long time. When a product requires repair or change frequently, then it is said to be unreliable. For example, if the use of plug‐in motor car engine lasts for long time, then its reliability is high; otherwise it is said to be unreliable.
1 Durability
How long the product serves? This is the most common property desired by the customers for any product. The customers want to purchase the products which serve them for longer lifetimes. Particularly, this dimension is applicable in automobile, electronics, communication products, etc.
1 Serviceability
How informal is it to get repair? Wear and tear is common in any product. The quality of any product is fit to the consumer if it is easy to get its service. The purchasing decision of the product is directly related to the serviceability of that product. For example, the mobile manufacturing companies normally provide after‐sales service of the mobile. Now if any problem is met to the user, then most of the companies provide the replacement of the mobile and some companies pronounce time for its repair as they send the mobile in the workshop, which may take weeks or even months to deliver it back to the customer.
1 Aesthetic
How beautiful the product looks? The look of the product plays an important role in determining the quality of the product. There are many factors that are directly related to the aesthetic of the product, for instance, the color, design, shape, model, style, packing, and such other sensual features. For example, in automobile industry the color and design of a vehicle play a significant role toward its quality.
1 Features
What extra functions the product can do? The customers have the tendency toward the articles having some extra features which others brands do not have. These added features are provided in addition to the original or main functions for which that article has been given. For example, if a mobile phone manufacturing company introduces a function of converting voice to text form while others do not have this function, then this is quality added function.
1 Perceived Quality
What is the repute of the product? Usually, customers are associated with those companies or products with which their past experience is pleasing and satisfying. Most of the customers attach the experiences of such other products while deciding to purchase or hiring a new one product or service. Normally the brand once selected urges the customers to decide for the next time. For example, consider an electronic company manufacturing a wide range of electronic goods like air conditioner, deep freezer, cooking range, washing machine, television sets, LEDs, mobiles, etc. If a customer uses a mobile phone of that company and he feels zero problem and fully satisfied