Introduction to Statistical Process Control

1.0 INTRODUCTION TO QUALITY

There are many variations in the definition of quality, but they generally fall into two categories. The first is design quality and the second is conformance quality. Quality of design is the quality which the producer or supplier is intending to offer to the customer. When the producer is making the quality of design of the product, he should take into consideration the customer's requirements in order to satisfy them with fitness for use of the product. If the quality of design does not reflect the customer's requirements, the product which the producer offers him would not probably satisfy the customer, even if it does sufficiently conform to the design. Quality of design is usually indicated by completeness and correctness of specifications, drawings, catalogues, and it is measured with fitness for use. While, quality of conformance is the level of the quality of product actually produced and delivered through the production or service process of the organization as per the specifications or design. When the quality of a product entirely conforms to the specification, the quality of conformance is deemed excellent. Specifications are targets and tolerances determined by the designer of a product. Targets are the ideal values for which production is expected to strive. In addition, tolerances are acceptable deviations from these ideal values recognizing that it is difficult to meet the exact targets all the time due to variability in material, machine, men and process.

Besides, quality is perceived differently by different people. Yet, everyone understands what is meant by “quality.” In a manufactured product, the customer as a user recognizes the quality of fit, finish, appearance, function, and performance. The quality of service may be rated based on the degree of satisfaction by the customer receiving the service. The relevant dictionary meaning of quality is “the degree of excellence.” However, this definition is relative in nature. The ultimate test in this evaluation process lies with the consumer. The customer’s needs must be translated into measurable characteristics in a product or service. Once the specifications are developed, ways to measure and monitor the characteristics need to be found. This provides the basis for continuous improvement in the product or service. The ultimate aim is to ensure that the customer will be satisfied to pay for the product or service. This should result in a reasonable profit for the producer or the service provider. The relationship with a customer is a lasting one. The reliability of a product plays an important role in developing this relationship.

According to Joseph M.  Juran (1951), he defined quality as fitness for use. Juran proposed the quality trilogy which is quality planning, quality control, and quality improvement to develop a universal thought process for quality. Quality planning is the process for preparing to meet the company’s goals. Both internal and external customers are identified and their needs are determined. Products and services are developed to fulfill these needs. Quality control is the process for meeting company goals during operations, and statistical process control techniques are the primary tools of control. Quality improvement is the process for breaking through to superior, unprecedented levels of performance. Juran stated categorically that waste must be identified and eliminated. Juran conceptualized the Pareto principle, which helps in identifying the vital few out of the trivial many.

But, Summers, Donna C.S (2010) believes that perhaps most complete definition of quality is as defined by Armand V. Feigenbaum. Feigenbaum has defined quality as a customer determination which is based on the customer’s actual experience with the product or service, measured against his or her requirements stated or unstated, conscious or merely sensed, technically operational or entirely subjective and always representing a moving target in a competitive market.

There are many others who contributed to quality improvement. So, we are encouraged to study current trends such as the Six Sigma approach and ISO 9000 certification. Quality improvement is an ongoing process, and the implementation of quality principles is not limited to industry.

INTRODUCTION TO STATISTICAL PROCESS CONTROL (SPC)

        According to James R. Evans (1991), SPC is a method of gathering and analyzing data to solve practical quality problems. The term statistical means that we will be drawing conclusion from numbers. Process refers to the fact that we will be concerned with a specific production process and its ability to produce output having consistent quality. Control means that we will monitor a process and adjust it when necessary so that it will perform in the manner that it is intended to perform.

Nowadays, each company employee must be committed to the use of effective methods to achieve optimum efficiency, productivity, and quality to produce competitive goods. Statistical process control (SPC), is a collection of production methods and management concepts and practices that can be used throughout the organization. SPC involves the use of statistical signals to improve performance and to maintain control of production at higher quality levels. It can be applied to any area where work is done. The statistical concepts that are applied in SPC are very basic and can be learned by everyone in the organization. Production workers must know how SPC applies to their specific job and how it can used to improve their output. Supervisors must be aware of the ways SPC can be used in their sections, be prepared to help their production workers utilize SPC, and be receptive to suggestions for improvements from the workers who are effectively using SPC. Managers must know how SPC can be used to improve quality and productivity simultaneously. They must create and maintain a management style that emphasizes communication and cooperation between levels and between departments. Their goal must be to develop a working atmosphere that maximizes everyone’s contribution to the production of competitive products.

        SPC is becoming the core for both quality improvement and quality maintenance. Important decisions made at the shop-floor level to process change decisions made by management involve SPC. Statistical methods and techniques, such as control chart analysis of a process or its output, are now being used extensively to make economically sound decisions. The process analysis leads to appropriate actions for achieving and maintaining a state of statistical control and for reducing process variability.

        SPC can improve quality by reducing product variability and production efficiency by decreasing scrap and rework. It can be used to monitor a process to determine when substandard items are about to be produced so that adjustments can be made to prevent the production of defective items. For each statistical application, such as control chart or histogram, there is an expected from pattern, and when the actual form of pattern differs from the expected, it is usually a signal that a problem exists. The potential problem must be investigated and eliminated. So, the SPC by itself will not improve quality, but only the appropriate action to its signals can improve and maintain quality.