Statistical Process Control

The premise in process control is that if the processes (chemical reactions, mechanical working by men and or machines) are operated within a tolerable range, the product produced will be of the desired quality. This assumes that the incoming raw materials are of the right quality to start with. The objective of process control is to set a proper procedure to work or shape the raw materials into finished goods and then monitor the process operating on the raw material frequently and any deviations from the said procedures should be corrected when required. Process control is nothing but the monitoring of the various physical variables operating on the materials and the correction of the variables when they deviate from the previously established norms.

But we know that most things have a variable component to them. The processes which operate on the raw materials will also have variations due to causes inherent in them or otherwise. In any case, the causes responsible for the deviation of the processes from the established norms have to be rectified. The causes responsible for the deviation could be such that they can be traced or spotted and therefore rectified and such that cannot be traced and rectified easily. In statistics, anything that we do not understand or we are not capable of understanding is called ‘random’.

The variations which are inherent in nature to a particular process and which are random since they are not traceable to any particular cause, are labeled to be due to ‘random causes’ or ‘chance causes’. For instance, a machine filling toothpaste in tubes may not fill all tubes with exactly the same amount of paste; there will be some variations. This is due to the inherent nature of the process. In Process Control, we would be concerned only with those causes which can be rectified i.e. the assignable causes. We can do nothing about the chance causes. But, anytime there is an assignable cause it is preferable to rectify it. The difficulty arising in this approach, of course is to know when a particular deviation in the process is occurring due to the chance causes and when it is occurring due to assignable causes.

Monitoring the Process:

We can control the process by (a) actually measuring the variables operating on the raw materials, or by (b) measuring the characteristics of the output product. When a number of variables are operating on the product, it becomes easier to monitor the processes by observing the quality of the product coming out of the processes, rather than monitoring the various variables operating. Moreover we are interested in the final quality of the product rather than in the process variables operating on the materials. Therefore approach ‘b’ which is to monitor the output of the process and based on that to make inference regarding the process operating on the raw material, is preferable. This is particularly so when a large number of variables are operating on the raw materials. The statistical process control would thus, seek to monitor the output of the processes and thus control the processes by locating the causes for the deviations (if any) and rectifying the same.

Specification Limits for the Output:

We do not say that the diameter of the shafts have to be 3 centimeters exactly. Rather we would say that it should be 3 centimeters plus or minus 0.002 centimeters. Similarly, we do not say that the pH of baby powder has to be 5.7 exactly. Rather we say that it has to be between 5.2 and 6.2. Every quality performance requirement is usually expressed within a certain range. Of the performance requirement in terms of quality of a product is called the specification range or specification limits. Therefore, when the quality of a particular product is being described, one usually refers to the appropriate range of performance of this product.

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