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Statistical Quality Control

Statistics means data collected to obtain reliable results. The science of statistics handiest this data in order to draw certain conclusions. Statistical techniques find extensive applications in quality control, production planning and control, business charts, linear programming etc.

Quality. It is a relative term and is generally explained with reference to the end use of the product. It is defined as fitness for purpose.

Control is a system of measuring and checking (inspecting) a phenomenon. If suggests when to inspect, how often to inspect and how much to inspect. It also incorporates a feedback mechanism which explores the causes of poor quality and takes correcting action.

A quality control system performance inspection, testing and analysis to conclude whether the quality of each product is as per laid quality standards or not. It is called statistical quality control when statistical techniques are employed to control quality or to solve quality or to solve quality control problems. It makes inspection more reliable and at the same time less costly. It controls the quality level of the outgoing products. The technique of S.Q.C.  Was developed in 1924 but it got recognition in industry only during Second World War. 

The fundamental basis of statistical quality control is the theory of probability. According to the theories of probability, the dimensions of the components made on the same machine and in one batch vary form component to component. This may be due to inherent machines characteristics or the environmental conditions. The chance or condition that a sample will represent the entire batch or population is developed by the theory of probability.

S.Q.C. uses three scientific techniques, namely:
(a) Sampling inspection
(b) Analysis of the data
(c) Control charting.

Sampling Inspection

A sample may be defined as the number of items or component parts, drawn from a lot, batch or population (for inspection purposes).

Inspection. Any item or component or product which is manufactured is required to perform certain  functions. The act of checking whether a component actually does so or not is called inspection. It separates defective component form non-defective ones and locates defects in raw material and flaws in processes which otherwise cause problem at the final stage.

Sampling. It is an act of drawing samples form a batch on random basis. The samples should be collected at regular intervals form the entire production run. The purpose is to obtain a sample truly representative of the lot.

Sampling Inspection.It is a technique to determine whether a lot or population should be rejected or accepted not eh basis of the number of defective parts found in random sample drawn form the lot. It the number of defective parts exceeds predefined level, helot is rejected.

Control Charts

Control Charts are based on statistical sampling theory. according to which an adequate sized sample drawn, at random ,form a lot represents the lot. Control chart is a (day-to-day) graphical presentation of the collected information. The information pertains to the measured or otherwise judged quality characteristics of the items or the samples. A control chart detects variations in the procession and warns if there is any departure forms the specified tolerance lines. It is dynamic in nature i.e. it is kept current and up-to-date as per the changes in processing condition. It depicts whether there is any change in the characteristics of items since the start of the production run.

Control Chats-Purpose

1.    It indicates whether a process is in control or out of control
2.    It determines process variability and detects unusual variation taking place in process.
3.    It ensures product quality level.
4.    It warns in time, and if the process is rectified at that time, scrap or percentage rejection cane be reduced.
5.    It provides information about the selection of process and setting of tolerance limits.
6.    It builds up the regulation of the organization through customer’s satisfaction.

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