Every production process is facing defects. Small defects or large defects, recurring or one-time defects, they are inherent to production processes. Today’s manufacturing challenge is to reduce these defects to decrease costs and increase customer satisfaction.

Below the difference is explained between defects and errors.


A defect is a non-conformity when comparing the result or output to the customer requirements. Defects can arise due to 3 different reasons:

  1. because of a difficult process
  2. because of variation in the process (environment, material, equipment, measurements)
  3. because of errors or human mistakes

Difficult processes can be simplified. Variance in the process can be reduced by means of Statistical Process Control (SPC) and errors should be addressed by implementing a poka-yoke.


An error is an inadvertent, unintentional mistake caused by a human/an operator, which can result in a defect.

Daily-life examples are:

  • Forgetting the keys in your car
    Time - Money - Safety - Reputation
    Time – Money – Safety – Reputation
  • Forgetting to pull out the plug
  • Running out of gas on the highway

Manufacturing examples are:

  • Forgetting to apply a felt
  • Forgetting to tighten a bolt
  • Applying the wrong component

Errors are a waste as they reflect loss of time, money, reputation and above all can have an impact on safety of the operators and the customer.

Why do errors occur?

As human being it is normal to forget something or make a mistake. We see this in our daily lives, so why would it be different in a manufacturing environment? Too often the operators are blamed for the mistake, but actually the system should be build in such a way that it is impossible to make the mistake in the first place.

Error classification

As a result, we can classify errors in subgroups.

  1. Processing or operation errors: a process operation is incorrectly executed. The process is not executed according to the standard. e.g. the bolt is not fully tightened.
  2. Missing part errors: a component is not assembled, e.g. only four out of five bolts are tightened.
  3. Setup errors: the wrong tools or machine settings are used. Incorrect adjustments are made, e.g. a bolt is tightened with an ordinary wrench instead of a QL-wrench.
  4. Omitted processing errors: a process operation is missed.
  5. Wrong part errors: an improper part is selected to assemble, e.g. a 5 mm instead of a 4 mm bolt is selected.
  6. Misassembled parts errors: a part is assembled incorrectly, e.g. a bracket is mounted upside down or a bolt is tightened upside down.
  7. Measurement or adjustment errors: a mistake was made in the measurement of a part, like measurements incoming parts, measurements on the machine settings, e.g. hole size is wrong.

Errors caused by humans can have many sub-causes like:

Untrained/Misunderstanding/Poorly informed
Untrained/Misunderstanding/ Poorly informed
  • Untrained operators or incorrectly trained operators
    > Solution: train and retrain
  • Misunderstanding by the operator: sometimes operators act too quickly before fully understanding the process and its requirements
    > Solution: training, check conditions before, standardize
  • Inexperienced operator: sometimes we put operators at work which do not have enough experience.
    > Solution: training, assign operators based on their skill-set, standardize
  • Inconsistent behavior by the operator
    > Solution: training, control of standard work
  • Poorly informed operator (e.g. due to shift change, sick leave, …)
    > Solution: ensure handover between operators, create back up
  • Inattentive or distracted operator
    > Solution: standardize work environment
  • Inadvertent or sloppy operator
    > Solution: create awareness, educate
  • Slow operator
    Lazy/ Slow/ Fatigue

    > Solution: train and practice

  • Laziness
    > Solution: educate, motivate
  • Fatigue (e.g. due to overtime)
    > Solution: ensure correct planning and level workload
  • Surprised operator
    > Solution: total productive maintenance and standardize work environment
  • Intentional mistake by the operator (sabotage)
    > Solution: Educate and change morale
  • Forgetful operator (poor memory): sometimes an operator forgets something when he is not concentrated.
    > Solution: control and alert

Note: Multiple sources were used for summarizing the content on this page. You can find these sources on the following page: References.