The Dirty Dozen - Part 1

- english human error human factors Dec 28, 2018


Ever looked back at an incident or accident to find the reason it happened and realised that you were stressed, tired or distracted and that is what caused the event? These are three of the ‘Dirty Dozen’ which human factors experts have identified as key contributory factors or precursors to incidents and accidents.

The term ‘The Dirty Dozen’ refers to twelve of the most common human error pre-conditions or precursors which lead to accidents and incidents. These twelve have been shown to influence people to make mistakes, errors or violations. The concept was developed by Gordon Dupont in 1993 and is now a key element to Human Factors in Maintenance training.

Note that these twelve are not the only factors which lead to mistakes, errors and violations, but they certainly give you a focal point to identify conditions where errors and violations are more likely to occur. Different domains or even subsets within domains like pilots, ramp crews, air traffic control within aviation have different ‘dirty dozens’ based on captured and analysed incident data. Unfortunately, no such data exists in diving which looks at factors which contribute to incidents and accidents. Consequently, the following is based on the original list and is not arranged in any specific order.

  1. Lack of communication.
  2. Distraction.
  3. Lack of resources.
  4. Stress.
  5. Complacency.
  6. Lack of teamwork.
  7. Pressure.
  8. Lack of awareness.
  9. Lack of knowledge.
  10. Fatigue.
  11. Lack of assertiveness.
  12. Norms.

While knowing what the issues are is a good start, it is important to develop and practice countermeasures to the Dirty Dozen thereby reducing the likelihood of an error or violation taking place, or if it does, the error is captured before it becomes too serious. As such, each topic will have concepts presented to help you prevent or mitigate the problem.

As you work through the Dirty Dozen, you will notice the interdependence e.g. lack of resources or miscommunication can increase stress and lack of knowledge can lead to reduced awareness.

As this is quite a long topic, I have split it into two parts, the second part being published mid-January 2019. 

Lack of Communication

Poor communication is often a major contributory factor in incidents and accidents and the need to improve communication is one of the key goals identified in my classes. Communication refers to both the transmitter (speaker) and the receiver (listener) as well as the medium and the systems used. Communications are often assumed to be person to person, but dive computers, SPGs and CCR controllers all communicate information to the diver. In all cases, it is often assumed that the information has been understood by the receiver, either because the receiver made some assumptions about missing information, or because the transmitter assumes the received heard/saw and understood the message. With verbal communication, it is common that only 30% of a message is received and understood.

Countermeasure. For a dive to be successful and all the goals/tasks/limitations understood, information must be passed between team members or instructor and student. Sometimes these messages are complex. If this is the case, write the message down or use checklists/worksheets. Verbal messages should be kept short with critical information contained at the start of the message and then repeated at the end. While assumptions are key to operating at pace, they should be avoided when it comes to critical information.   


Distraction is part of our modern lives. We are unable to perceive and process the vast amount of information available to our senses. As such, we filter and discard what we don’t think is important or relevant. Even worse, if something comes to our senses which looks really ‘interesting’ we start to focus on that and forget what we were doing before. Those distractions could be conversations, other people doing things around you, environmental issues like noise, wind or the cold, along with problems that need to be solved straight away for safety reasons like a piece of dive equipment falling over or about to blow away. Distractions can also be emotional or psychological in nature like family issues or how you’re going to pay for the trimix bill!

Psychologists believe that distractions are the number one reason for forgetting things and if you’ve forgotten to do something unless something triggers the thought you forgot, it will remain forgotten! We have a tendency to think ahead (in many cases that is a good thing), but this means that when we return to a task, we think we are further ahead than we really are.

Countermeasure. Two ways to deal with distraction are to look at preventing it and then mitigating it.

  • Prevention. Let your team or students know that you are doing something critical (like assembling your camera or CCR) and that unless it is really urgent and can’t wait they should hang back and come and find you afterwards.
  • Mitigation. If possible, finish the task and then go to the ‘distraction’. However, if you are going to be drawn away from the primary task, put a mark or sign as to where you are in a sequence that is really obvious to draw your attention back to that. I would also recommend you go back two or three steps in any checklist as we have a tendency to overestimate progress.

Lack of Resources

If we don’t have the correct parts or tools to complete the task, as divers we often try to improvise with what we have. In many cases, it is fine and nothing happens. However, unless we understand the criticality of the components or the task at hand, then disaster can strike. Resources also refer to people, skills, experience, knowledge etc and a lack of these can interfere with our ability to complete the task safely and effectively. Even more important is that if the task is successful without using the correct resources, normalisation of deviance can set in (see Norms).

Countermeasure. When the proper resources are available and to hand, there is a greater chance that the task will be completed more effectively, correctly and efficiently. This might mean a small expenditure in the short term, but lead to safer divers and potentially saving money in the longer term when the botched job doesn’t need to go to a service or repair tech.


Stress can be a good thing, eustress, or it can be a bad thing, distress. Distress comes in two forms which impact our performance, acute or chronic. Acute distress comes about from the real-time demands placed on our bodies and minds from dealing with unexpected events, working under time-pressures or with inadequate resources. Chronic distress comes from long-term demands like job security, family issues/relationships or illness and can impact performance by taking up a chunk of our processing power which means awareness is reduced and decision making is more likely to be flawed. Interestingly, if there isn’t enough stress to keep our interest levels up, then we start to become complacent about the situation (see the next topic below).

Countermeasure. Early signs of stress include changes in personality and moods, errors of judgement, lack of concentration and poor memory. Individuals may notice difficulty in sleeping and an increase in fatigue, their eating habits might change with comfort eating more likely or not eating at all. Longer-term signs of stress include susceptibility to infections, increased use of stimulants and self-medication, absence from work, illness and depression. Distress is a very personal thing and therefore external cues/clues aren’t always obvious.

In terms of diving, the activity of diving can reduce stress and provide a way to ground ourselves in reality, forgetting about everything on the surface. Just be aware, that significant stress can lead to reduced and dangerous performance, like the diver who left his scrubber out of his rebreather because of massive stresses topside. Fortunately, he realised something wasn’t right and abort the dive just before they entered the cave system for a 4-5 hour dive…


Complacency can be described as a feeling of self-satisfaction accompanied by a loss of awareness of potential dangers or threats. It often comes about when conducting routine activities that have become habitual and the perception of risk of getting something wrong is low. This reduces the attention or vigilance paid to the situation and important signs/signals/cues will be missed and due to cognitive biases, the diver only sees what they expect to see. Research has also shown that immediately following a high-stress situation, complacency can be encountered due to the relief felt in resolving the situation.

As highlighted above, too much stress can be a bad thing when it comes to performance. However, if there isn’t enough eustress to keep the attention focused on the topic at hand, then complacency can set in. As such, if a low-stress environment is being encountered, combined with fatigue (like a liveaboard!), then additional vigilance must be applied.

Countermeasure. Avoid working from memory for critical tasks e.g. building and final-jump checks on a CCR. Look for reasons why something shouldn’t be right. e.g. when building a camera rig, make sure all of the controls work and the leak test is correct before getting in the water. Leaving something to the last minute means that time pressures will lead to sub-optimal configurations being accepted. Finally, using team-mates to cross-check and keep the team accountable is a good way of reducing complacency. However, without clear standards and acceptable behaviours/skills, it makes it hard to challenge others.

Summary (of part 1)   

The Dirty Dozen is a simple list of the most prevalent error-producing conditions for a particular domain. The list above is based on the original list from 1993 (and codified here - but many other variants are out there.

When you look back at your dives, especially those which didn’t go to plan, consider these topics and work out what you will do differently next time. Be careful not to only focus on these, as there are many more factors which can lead to accidents and incident happening, but they will give you something to shape your thinking.

The most effective way of reducing incidents and accidents is to design out the problem by hardware or work design. In some (most) cases, divers can’t do much about the design. The next step is predicting when issues are more likely. Understanding ‘The Dirty Dozen’ can heighten awareness of when failures are more likely.

The next part will be published mid-January 2019.

Gareth Lock is the owner of The Human Diver, a niche company focused on educating and developing divers, instructors and related teams to be high-performing. If you'd like to deepen your diving experience, consider taking the online introduction course which will change your attitude towards diving because safety is your perception, visit the website.