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June 2019    Download the Entire Issue (PDF) Available to the Public Vol. 45, No. 6   RSS Feed for Undercurrent Issues
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What’s Your Real Risk of a Diving Death?

the ratios versus the reality of “what happens if . . .”

from the June, 2019 issue of Undercurrent   Subscribe Now

The risk of a fatality in diving has been stated as:

  • * 16.4:100,000 divers (Diver Alert Network's figures)

  • * 14.4:100,000 divers (British Sub-Aqua Club's figures)

  • * 0.54:100,000 dives (BSAC member dives)

  • * 1.03:100,000 dives (non-BSAC member dives)

But what do those statistics actually mean? I can tell you they mean nothing to most people -- we don't deal with risk in the real world like that. Numbers don't have the same emotional relevance as stories about dive deaths, so they don't stay in our heads that long. Furthermore, most decisions are informed by emotion, not logic. So let's use a visual example of the apparent irrationality of perceived risk in scuba diving.

A colleague of mine did a "Discover Scuba Diving"-type dive while on a Caribbean cruise. Her only diving experience prior to getting into the ocean was in the pool of the cruise liner she was on. Her first dive was to 100 feet on a deep wall advertised as a shark dive. The biggest concern for her and the other divers wasn't the very deep water they were over, it was the sharks they had gone down to see. She remembers getting down to 100 feet and swimming around, completely oblivious to her gas consumption, depth or decompression obligations that might come up. It took the guide several goes at grabbing her attention and getting her to ascend, as she was getting very low on gas. If the guide hadn't done that, she would likely have completely run out of gas because she was so enthralled by where she was. She had gone on the dive because she knew the operators wouldn't have put her into a dangerous situation with the sharks, so she completely trusted them to make sure the diving was safe and didn't think about that risk.

Probabilities Are Irrelevant

Ironically, the risk of dying from a shark attack is phenomenally small, with only five to 15 fatalities per year. But the statistical risk of dying during a diving trip is much higher. However, in both cases (shark or diving), these risk numbers are meaningless because the outcome ends up as either 1 or 0, and you only find out after the event. You cannot be a fraction of dead. That might appear a rather obvious thing to say, but consider the normal risk equation people use.

Risk = the probability or likelihood of the event occurring x the consequences (loss or benefit)

This non-diving example makes the point clearly. Suppose that on a certain portion of a highway, radar is mounted with the intent of catching speeders. If you're 20 miles per hour above the speed limit, you can expect a fine of around $100. It's known from statistics that on that portion of the road, only one driver out of ten on average gets caught. The above formula would suggest that anyone who is speeding is risking 1/10 X 100 = $10. This is, of course, senseless. If you get caught, you pay $100; if you don't, you pay nothing. In other words, if you drive fast, you're risking a $100 fine, not a $10 one. Try arguing with a policeman that your fine should be $10.

In that simple example, the cost was easy to estimate -- $100. However, the p=1/10 is irrelevant. Things either happen or they don't.

Herein lies the problem with risk management in diving, or any high-risk sport. We don't know the probability or likelihood of the adverse event because we don't know how many divers there are, how many dives those divers complete, or the depth, time or any other variable that impacts the likelihood. In addition, we have no way of calculating the interaction of the multiple factors that could lead to a fatality. So the likelihood is an unknown, and as you can see from the speeding example, it doesn't really matter anyway.

And the consequences? What do you measure those by? Consider that if the risk is such that one in 200,000 dives ends in a fatality, you can't be 1:200,000th dead. So at an individual level, the consequences for a diving fatality are expressed as either a one or a zero.

If you use statistics, the risk is a very small number. This low-risk factor is how diving is sold as a safe sport with benefits that massively outweigh the consequences. However, the emotions massively contribute to the biases we are subject to when making decisions that involve risk. That risk could be related to a 50-foot openwater dive, a 500-foot rebreather dive, or a dive that goes three hours back into a cave system where no one has been before. Our risk management (in diving) is based on heuristics, biases and emotions, not on logic. It is individual, contextual and very variable.

How Our Diving Decisions Can Be Totally Wrong

Psychologists Daniel Kahneman and Amos Tversky won the Nobel Prize in Economics for looking at decision-making, especially apparently irrational decisions, and coming up with a term called "behavioral economics," in which they looked at how much it was worth to make a decision. They determined we don't logically weigh up every decision we make, because we don't have enough time or mental energy to do this, so we substitute real data for mental models of what is likely to happen -- even if we know this is wrong.

So, say I flip a coin 10 times and heads comes up each time; what do you think the 11th flip will be? Most people will say tails, because they don't believe that 11 heads in a row is likely to happen, and because we have never heard of anyone doing that. Since such a situation is so unlikely, it is uncertainty that we are managing most of the time, not risk. This ability to quickly grab something from our memory and use it to inform our decision-making is called availability bias.

You dive to 130 feet on air because it is part of your dive agency's training and you are certified to do it, even though you've never been that deep during the training. You continue to dive to that depth because you've never heard of anyone having an emergency and not being able to resolve it due to narcosis, thinking that muscle memory will help solve the problem. People suffering from narcosis are not able to undertake higher brain functions, which include memory and complicated problem solving, so why would they remember the problems they had? Furthermore, if you realized you did something that nearly ended with you dead, and that deep air is recognized as "not a good thing to do," how many people would you tell? Consequently, your experience is not known by the rest of the community.

Another bias that is working against "logic" is outcome bias. If nothing bad happened, but you got the benefits you were looking for, it is often considered a good decision. This bias can lead to normalization of deviance over a period of time where new baselines are set, further and further from "safety."

Something else to consider when it comes to outcome bias. What is the worst that could happen on a dive? Death. Maybe. But what about paralysis and being stuck in a wheelchair caused by decompression sickness? Wouldn't that potentially cause you more stress? Perversely, I have heard of a diver out of gas holding a stop, rather than ascending, because his computer was stating he still had deco to do. It was a dive that ended with 10 minutes of unplanned deco, but his buoyancy control was not good, so each time he dropped below 20 feet, the computer stopped counting his stop time. In the end, he had completed something like 30 minutes of stops in the area of 20 feet before he ascended. He didn't know what the expected deco was for that situation, and couldn't deal with the uncertainty.

Risk management in the context of diving is about dealing with uncertainty. However, we often confuse a world of uncertainty with one of known risk. This has been described as the calculable-risk illusion.

We don't logically weigh every decision we make, because we don't have enough time or mental energy, so we substitute real data for mental models of what's likely to happen -- even if we know this is wrong.

Say a diver was on a liveaboard a long way from land, like Cocos Island, and her trip was halfway done. She surfaced from a dive and realized she was suffering mild symptoms of DCS. So she breathed 100 percent oxygen on the boat and felt "fine" the following day. She went diving again, but this time, when she surfaced, things were much worse -- she was paralyzed from the waist area down and couldn't use her legs or bladder. While she had insurance, there is not much you can do for an immediate response when 30 hours offshore. One of the most important things was to release the pressure on the bladder, so her husband had to do a DIY catheterization with guidance from a doctor over the satellite phone using whatever they could find on the boat. Their liveaboard transferred them to another boat in the area that was going back, and the diver ended up with something like six weeks of hyperbaric treatment before being able to fly home.

Why is this relevant? A friend of mine posted that his buddy was being treated in a chamber for a serious bend, while one of his friends posted that he had recently had a skin bend on a 165-foot dive, then went diving the following day to see if the bend would clear. It didn't, so he took the third day off.

"What if..." is a risk-management question. Many divers think about transferring the risk they face to an insurance company so that medical coverage can be sorted or compensation provided. However, if the risk ultimately materializes, someone else might be paying for it, but it is you who will be suffering. That doesn't just pertain to injuries, but what about instructors and litigation?

How to Improve Your Risk Management in Diving

First, recognize that you are truly fallible, and that "decisions in the moment" are going to be made with emotion, rather than logic, at the fore. This is one of the reasons why checklists are so effective -- they force people to slow down and follow a logical process rather than get "sucked up in the moment."

Second, have a plan and brief it. This allows divers taking part to understand beforehand what is going to happen and what the contingency plans are. It is much better to discuss the "what ifs" on the surface without having to plan and execute the resolution at the same time. This is one reason why team diving (not buddy diving) is safer than solo diving.

Third, practice the art of the debrief. This is really hard to get going, but when it does, you will improve your performance and safety because you will be building up a bank of experiences that you can refer to and inform your availability bias, and subsequently your decision-making.

Debriefs don't have to be long and torturous, they can be as simple as:

  • "What went well?"

  • "Why?"

  • "What do we need to improve on?"

  • "How are we going to fix it for next time?"

  • "What was the biggest risk we took on that dive?"

But you need to be specific in your answers. In the same way that risk numbers like one in 200,000 are meaningless, debriefs that talk about generalities are also pretty much useless. It is the context and the detail that is needed to create emotion, and it is the emotion that causes the "mental stickiness."

Finally, consider sharing your successes and failures with the diving community. The learning that comes from numbers is limited, whereas the learning from an emotionally-charged and context-rich story is much better.

Gareth Lock is director of risk management for the dive training organization Global Underwater Explorers, and owner of The Human Diver, a training and coaching company focused on developing high-performing divers, dive instructors and related teams. You can read more of Lock's diving articles at

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