Quality Control: Prevention or Detection?

Prevention or Detection as Quality Control

Prevention or DetectionAn age-old question: Quality Control: Prevention or Detection? Estimates show that it is ten times more costly to correct a problem than it is to prevent it. With this kind of savings, companies can reduce prices, expand their R&D, and increase profits at the same time. Changes to a product’s characteristics are merely a reflection of changes in input. Therefore, the key to preventing defects in a product is to monitor and control all aspects of its production.  This brings us to an exploration of processes.

Definition of a Process

Most people think of a process as a machine or task, but it is more complex than that. A process is the entire system of machines, raw materials, people, procedures, environment, and measurements, used to make a product. The figure at right  illustrates this concept. You can define a process to suit your needs. If your interest is the production of plastic buckets, you can define the process as the bucket, machine, and the plastic. Or you can expand the definition to include all production in the plant as well as external factors that impact plant operations.


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A process has inputs, actions, and outputs. Each input has a source or supplier and each output has a customer or a user.  A process is at the mercy of its inputs. If the input changes, the change reflected in the output. The most common way to deal with this is to change certain inputs to compensate for unwanted changes in others. However, since each factor can vary greatly, this method is time-consuming and costly. If you reduce changes or input variation, the process will need fewer corrections, the products will be more uniform, and less work will be needed to produce the same output

As for prevention vs. detection? It is far more effective to determine which inputs affect the process most, and control them than to compensate for changes in these inputs.

Who is the customer?

So let’s carry this a step further.  Every process has customers. They are the people who receive the output of the process. They may be the next process in the plant or the people who buy the end product. The goals of a quality program are to define the customers for each process, and to make sure their needs are met. This is where the idea of specs clashes with our thinking. Rather than asking what the customer wants, specs ask what is the worst the customer will accept.


This content is adapted from Zontec’s The Book of Statistical Process Control.  You can download a free copy here.