DCS--Playing the Odds

[Kevrumbo]

In making the diagnosis of the cause of illness in an individual case, calculations of probability have no meaning. The pertinent question is whether the disease [or disorder] is present or not. Whether it is rare or common does not change the odds in a single patient. ... If the diagnosis can be made on the basis of specific criteria, then these criteria are either fulfilled or not fulfilled. — [A. McGehee Harvey, James Bordley II, Jeremiah Barondess: p.15 in Harvey, A. M.; et al. (1979). Differential Diagnosis (3rd ed.). Philadelphia: W.B. Saunders]

There's a dilemma in considering the low statistical incidence of DCI in the general diver population versus for example -the high morbid consequences of suffering a type II DCS/AGE in specialized technical decompression diving, with a PFO as a possible contributing -or "associated" condition. So you have a low probability event vs a severe health & welfare outcome if you unluckily suffered such an occurrence.

IMO, a better understanding the statistical significance along with your own health & physical history is a more practical perspective in applying a continuum of personal risk management: In other words, the chances are low of contracting DCI in regular NDL recreational diving, but on the other hand, would you do 3hr runtime, saw-tooth profile, decompression cave dives in hypothermic stress if you knew you had an underlying PFO condition to begin with?

 

[mi000ke]

In making the diagnosis of the cause of illness in an individual case, calculations of probability have no meaning. The pertinent question is whether the disease [or disorder] is present or not. Whether it is rare or common does not change the odds in a single patient. ... If the diagnosis can be made on the basis of specific criteria, then these criteria are either fulfilled or not fulfilled. — [A. McGehee Harvey, James Bordley II, Jeremiah Barondess: p.15 in Harvey, A. M.; et al. (1979). Differential Diagnosis (3rd ed.). Philadelphia: W.B. Saunders]

This flips the case, working backwards from effect/outcome (viz., DCS) to cause. It is true, if a particular individual has DCS, all the stats in the world won't make it go away. When you have an equifinal causal chain (many causal paths for an outcome) working backwards can be very difficult if not impossible - again because many causes could be latent or interacting with one another, or plainly unknown.

Statistics are a proxy for causal knowledge, when used going in the other direction to predict outcomes/effects from "causes." If we knew and could specify all of the causes and the specific process(es) by which they led to specific outcomes, we could in fact determine whether or not a specific individual would get DCS in a specific context. That level of knowledge is essentially unattainable, so we substitute statistics - which by their nature are an abstraction that leaves out all the causal things we don't know. And that's the beauty as well. We can operate with some fuzzy level of knowledge within some "confidence interval" and be right most of the time regarding something we do over and over. But any one instance could result in any outcome. So even for a specific individual who dives exactly the same way all the time (if that were possible), if we somehow could know and determine that the probability of getting bent for that person diving that way were .001%, there is still a chance on any dive that that person could get bent (and this is the limited case where probability would hold - meaning in this case over 10,000 dives you will *probably* get bent once). But looking back on any dive after the fact, the probability was either 0% or 100%. Not much help.

 

[MargaritaMike]

IMO when it comes to diving, given the number of variables in play, the only thing that percentages show is what HAS happened. To say the same percentage applies to each individual going forward is a mis-application of historical occurences. All the percentage show is for the divers who have or had DCS, their number adds to the total cases of DCS since the beginning of recorded statistics of reported cases. How many people have had minor symptoms of DCS which over time were cured and they didn't even know they had it that didn't get reported? As I said earlier, I am due to get DCS on my next dive so I will dive with that in mind and keep my profile such that I minimize the adverse effect of bubbles. If I don't get it, then I did everything right. If I do, then my statistics are up and/or I did something wrong.

Cheers - M²

 

[Kevrumbo]

IMO when it comes to diving, given the number of variables in play, the only thing that percentages show is what HAS happened. To say the same percentage applies to each individual going forward is a mis-application of historical occurences. All the percentage show is for the divers who have or had DCS, their number adds to the total cases of DCS since the beginning of recorded statistics of reported cases. How many people have had minor symptoms of DCS which over time were cured and they didn't even know they had it that didn't get reported? As I said earlier, I am due to get DCS on my next dive so I will dive with that in mind and keep my profile such that I minimize the adverse effect of bubbles. If I don't get it, then I did everything right. If I do, then my statistics are up and/or I did something wrong.

Cheers - M²

More importantly, what are the chances that you'll find a Recompression Chamber to promptly treat you 24/7 & 365 for DCI (from simple type I DCS -to worst case AGE, near drowning in full arrest)?

[Here in SoCal, it's 100%]

 

[Akimbo]

Or to quote my old man " your not in a position to make a good decision with your head up your a**".

Maybe that's why everyone keeps recommending that I install a viewport in my belly button so i can see where I'm going?

 

[BRT]

I'm not sure getting bent is a yes or no thing. I dive with fishermen in Mexico. They dive profiles none of us would dive. They have learned how long to decompress based on how they feel. I had a couple of spare tanks of nitrox I let them use one trip to decompress on because I couldn't go with them that day. Two of them approached me that night and told me how much better they felt after using the nitrox to decompress. Now they use it for that purpose all the time. My guess is they will eventually lower decompression time until they start to feel bad again. They are not "bent". They just don't feel all that good.

 

[boulderjohn]

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.

"you still aren't more likely to get bent on dive 2000 than you were on dive 1000 (all other things being equal)." I am not sure what you mean by this, because the likelihood of all other things being equal is about nil. If dive 1000 was to 150 feet and violated all standard decompression profiles, and if dive 2000 was to 15 feet for 15 minutes, I would say you more more likely to be bent on dive 1000. That was my point--all other things are NOT equal in diving. Each dive has its own characteristics and its own individual level of risk.

While it is true to some extent that the more you dive, the more likely you are to be bent, I would say that is only true if those extra dives are more risky than others. Would you say that Joe Blow's 300 dives, all at 20 feet for less than 20 minutes, put him more at risk for DCS than Jan Doe's 30 dives, all at the edge of NDLs?

 

[w3dge]

"you still aren't more likely to get bent on dive 2000 than you were on dive 1000 (all other things being equal)." I am not sure what you mean by this, because the likelihood of all other things being equal is about nil. If dive 1000 was to 150 feet and violated all standard decompression profiles, and if dive 2000 was to 15 feet for 15 minutes, I would say you more more likely to be bent on dive 1000. That was my point--all other things are NOT equal in diving. Each dive has its own characteristics and its own individual level of risk.

While it is true to some extent that the more you dive, the more likely you are to be bent, I would say that is only true if those extra dives are more risky than others. Would you say that Joe Blow's 300 dives, all at 20 feet for less than 20 minutes, put him more at risk for DCS than Jan Doe's 30 dives, all at the edge of NDLs?

You are of course correct that a higher risk profile dive will result in a higher probability of DCS - that much is obvious. What I was trying to illustrate is that this notion that some people seem to get from reading statistics is that your chance of getting DCS increases with every dive you do until you succum, is incorrect.

 

[Storker]

What I was trying to illustrate is that this notion that some people seem to get from reading statistics is that your chance of getting DCS increases with every dive you do until you succum, is incorrect.

For a simpler illustration: If you roll a die ten times, the probability of scoring a six at least once, is 84%. Still if you've rolled the die nine times without scoring a six, the chance of scoring a six on the tenth attempt is still only 17%. Because what has happened in the past has no bearing on the future.

 

[Neilwood]

For a simpler illustration: If you roll a die ten times, the probability of scoring a six at least once, is 84%. Still if you've rolled the die nine times without scoring a six, the chance of scoring a six on the tenth attempt is still only 17%. Because what has happened in the past has no bearing on the future.

We can take that probability even further in the case of DCS where we can in effect alter the shape of the die.

By choosing to dive conservatively we are effectively loading the dice in our favour but of course there is still no guarantee of not throwing a six even with loaded dice.