Defect Prevention - We first have to observe a defect before we can prevent?


Dan De Yarman

Defect Prevention

What are some easy, yet effective defect prevention methods?

Don't we first have to observe a defect before we can attempt to prevent a similar one? Is it similar to the difference between corrective vs. preventive action; where corrective action is performed to correct a specific problem, and preventive action is performed on similar jobs(procedures, etc.) to prevent the problem from occurring?

Thanks for your help,


Dan De Yarman

I've heard the term before, but I'm not sure what it is or how to use it. Could you please describe what it is and how to use it? I guess I'm still not up to speed on all the theoretical aspects of quality control/quality assurance.

Thank you Marc,


Laura M

Lot's of examples come to mind from my mmanufacturing days. Errorproofing is another term.
- If a part can be designed non-symmetrical to avoid accidentaly misassembly. (upside down or backwards - offsett locating pins)
- If not by design, then is there a feature on the part you can use to design the nest so it can only be assembled one way.
- Color coding 2 different part numbers that are similar for correct assembly, and using light detection to verify the right part. Or better yet, can the designs be reduced to one?
- Having rejects pass through a reject chute (with detection verification) before the stand will cycle again.

Haven't referenced the pdf files as yet, may be redundant...if I think of more, shall I let you know? Of course to be considered "prevention" these activities should take place before a reject is produced, but are just as useful as corrective action.


Fully vaccinated are you?
ISO 9001 requires that you have procedures for identifying and eliminating the causes of actual or potential nonconformities in your products, processes, or the quality system.

Product and service nonconformance reports are the source of information leading to the corrective action that you should take to prevent the same or a similar problem from recurring. Situations may include the following:

Failures, malfunctions or nonconformities in incoming materials, processes, tools, equipment or facilities needed to produce your product or deliver your service, including the equipment and systems you use;
  • Inadequate or non-existent procedures and work instructions;
  • Non-compliance with procedures or work instructions;
  • Inadequate process control;
  • Poor scheduling;
  • Lack of training;
  • Inadequate working conditions;
  • Inadequate resources, (human or material);
  • Inherent process variability.

These conditions may be found by reviewing:
  • Inspection and test records;
  • Nonconformance reports;
  • Observations during process monitoring;
  • Audit observations;
  • Field, service, or customer complaints;
  • Regulatory authority or customer observations;
  • Observations and reports by your personnel;
  • Sub-contract problems;
  • Management review results;
  • Inherent process variability.

The same causes and conditions may indicate where preventive action is needed and where patterns or trends that may indicate the potential for nonconformities should be investigated. The degree of corrective or preventive action taken depends upon and is directly related to the risk, size and nature of the problem and its direct effects on product quality.

Corrective and preventive action reports should be part of the management review process.

Corrective action

Corrective action is taken to eliminate the cause of an existing nonconformity or other undesirable situation. There is a distinction between "correction" and "corrective action". Correction refers to repair or rework as is done to eliminate a nonconforming product situation whereas corrective action is taken to prevent recurrence of the situation.

Your procedures should describe how to implement corrective action, how it should be carried out and the effectiveness of the results verified.

It is important to implement procedures to deal with nonconformities discovered in any products that have already been delivered. You must determine whether the nonconformity is an isolated or a repetitive problem, and any actions to be taken, if necessary.

Corrective action to eliminate the cause of a nonconformity is not necessarily required every time or for isolated incidents of a minor nature, but a periodic analysis of nonconformance reports should be done to identify opportunities for process improvement.

Preventive action

It should be noted that corrective action is taken after nonconformities are identified. Preventive action is taken when a potential nonconformity is identified as a result of the analysis of records and other relevant sources of information, such as the following:
  • Statistical process control documents;
  • Customer complaints;
  • Review product, process and quality system information,
  • Risk analysis and risk assessment of products and processes.

Records relating to the product performance should be analyzed regularly, to detect trends and to identify areas of risk that may lead to nonconformities. The analyses should also determine how to prevent any identified potential problems.

Information on preventive action taken must be part of your management review to maintain and improve your quality system.

Preventive action is taken to prevent occurrence whereas corrective action is taken to prevent recurrence of a situation or nonconformity. Preventive action is not necessarily required for all the potential nonconformities you have identified, but it should be considered if there are opportunities to improve your quality management system.


[This message has been edited by Marc Smith (edited 27 June 2000).]


The easiest defect prevention approach I have used (for mass production type manufacturing is to:
1. Record all defects part numbers, quantities and reason for defect. (You probably already record this info or similar).
2. Each month, group all the defect information by "reasons for defect" and the number of occurances. A pareto chart is the ideal. It should show what defect type occured the most.
3. Talk with all relevant people associated with the process causing the highest defect for the month, to determine the root cause. this includes engineers, operators, supervisors, subcontractors, etc. 4. Develop an implementation plan to prevent the defect problem from recurring. (This must eliminate the root cause).
5. Future months pareto charts should show a reduction in the defect.
6. You know when you are winning when the smallest reason for defects become your highest reason.
I have tried this system on chrome/gold/zinc plating lines with enourmous success.

barb butrym

Quite Involved in Discussions
here is a poke yoke example from real life

Those little white plastic tripods that sit in the middle of the take out pizza to keep the box from collapsing......and messing up the cheese.

Proof Reader

Another practical example would be your shoes. It is shaped in such a way that you'll have to insert your right foot in the right pair and the left foot in the left pair. It will be hard to do it the other way around, and it will painful to do so.
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