Some raw materials are sensitive to humidity; this valve sticks and is more demanding than its supposedly similar neighbor; that pump has broken down; this operation sometimes occurs at night and sometimes during the day, sometimes when it’s hot, other times when it’s cold, sometimes our co-worker is tall, sometimes he is short, we may be under pressure time-wise, etc.
Thanks to their experience and abilities, operators will recognize these variations and try to adapt their way of working accordingly. Sometimes, they will detect that the situation is very far from normal and will seek help from their co-workers or their managers. Production occurs only because each person manages many sources of variation while executing his tasks, with expertise acquired through experience.
Source: The ICSI “Safety Culture” working group (2017). Safety Culture: from Understanding to Action. Issue 2018-01 of the Cahiers de la Sécurité Industrielle collection, Institut pour une Culture de Sécurité Industrielle (ICSI), Toulouse, France.