DCS--Playing the Odds
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Bob DBF
Contributor
Well that's just a waste of experience.
Or to quote my old man " your not in a position to make a good decision with your head up your a**". You can guess who he was talking to. It was hard to take at the time but I miss him and his peers now.
Bob
------------------
And get off my lawn!
Gee, I didn't think it was that many.... Hmmm! I learn something new every day. Thanks.
Cheers - M²
Whatever the probabilities are for a given diver, given a sane diving habit, since there is a finite number of possible dives in a finite life time, the proabability of DCS is not 1.0. For some it could be close to 1.
Using a statistic for road casualties - in 2015 there were approx 180,000 road casualties in the UK. Given a reported population of approximately 65million, that gives a risk of 0.28% of injury. Looking at that stat, I could say that I have a 0.28% chance of an accident per year therefore if I were to live to 100, my likelihood of an accident is 28%. Clearly wrong as it takes no account of my actions or circumstances. If I were to drive at unsafe speeds every day, I would have a far greater likelihood of an accident than if I lived on an island with no roads or only had a car pass me once every day.
For the vast majority of divers, we have no idea how close or far we are from suffering with DCS on any particular dive. I might be a fraction or a country mile from it but without a medical team on hand to assess me, I have no way of knowing. Am I bubbling and if so, are they within safe limits? What are the safe limits for me on that particular day? They might be hugely different for my buddy who has just carried out the same dive. I might be safe on one day with a particular profile but on another with what seems like a minor change I might suffer DCS.
The best that any algorithm can give is a statistical prediction that the profile, in the vast majority of cases, will or will not cause DCS. No one can predict with absolute certainty whether any dive profile can guarantee being DCS free. By that token no one can guarantee that after a certain number of dives I will have suffered DCS. What I can do is lower my risk by staying fit and diving conservatively.
For the vast majority of divers, we have no idea how close or far we are from suffering with DCS on any particular dive. I might be a fraction or a country mile from it but without a medical team on hand to assess me, I have no way of knowing. Am I bubbling and if so, are they within safe limits? What are the safe limits for me on that particular day? They might be hugely different for my buddy who has just carried out the same dive. I might be safe on one day with a particular profile but on another with what seems like a minor change I might suffer DCS.
The best that any algorithm can give is a statistical prediction that the profile, in the vast majority of cases, will or will not cause DCS. No one can predict with absolute certainty whether any dive profile can guarantee being DCS free. By that token no one can guarantee that after a certain number of dives I will have suffered DCS. What I can do is lower my risk by staying fit and diving conservatively.
That is what the NDL number on your computer's screen is. Trust the computer and you'll only have a 23.45679% chance of getting DCS, like all those DSC cases who did follow their computers and got bent. But overall your chances of DCS would be below .02%. Unless you actually get bent in which case it's 100%.
Numbers: there's so many of them and stuff.
w3dge
Contributor
I guess a little of the gambler's fallacy comes in also. Ignoring the actions a diver can take to change their risk profile, as already discussed in this thread, and assuming a fixed probably of getting bent, you still aren't more likely to get bent on dive 2000 than you were on dive 1000 (all other things being equal). Yet the more you dive, there more likely you are to get bent.
Interesting interaction...
Interesting interaction...
mi000ke
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As one who had to deal with stats and causality a lot in a former life, I think the key take-away of this great post is that statistics in non-controlled situations where there are many variables - some identifiable, some latent, some controllable, other not, some objectively quantifiable, others subjective, and on and on - are not very meaningful.
Without measuring cause and effect (DCS) within some objectively measurable and controllable context (like the physiology of the diver) similar to drug trials, we're just overthinking the math.
And as with any "at risk for..." situation, you do what you can to mitigate your exposure to risk and hope for the best. Be conservative with your NDL, do a safety stop, ascend slowly, take long surface intervals, limit the number of dives per day, use nitrox with air tables - whatever.
And if you still get bent, well at least you tried.
Without measuring cause and effect (DCS) within some objectively measurable and controllable context (like the physiology of the diver) similar to drug trials, we're just overthinking the math.
And as with any "at risk for..." situation, you do what you can to mitigate your exposure to risk and hope for the best. Be conservative with your NDL, do a safety stop, ascend slowly, take long surface intervals, limit the number of dives per day, use nitrox with air tables - whatever.
And if you still get bent, well at least you tried.
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