DCS--Playing the Odds

[jeff h]

How is that different to how he should behave in the other scenario?

The other scenario is they don't take care of themselves, run aggressive profiles and are at a higher risk for DCS.

 

[jeff h]

@KenGordon - I think what boulderjohn is proposing is that we do have some control over what group we are in if we take better care of ourselves. Although, there are still some other physiological factors like PFO (paten foramen ovale) that will predispose certain individuals to a higher risk group.

 

[Storker]

The scientists who developed the PADI tables tested divers using Doppler bubble imaging to detect bubbles in the blood. They were very surprised to see what a difference there was between divers. Some developed bubbles much more easily than others.

About a dozen of my clubmates participated in a hyperbaric medicine study some two to three years ago. The primary response factor was bubbles determined by ultrasound. The values were so over the whole map that the scientists weren't able to draw any conclusions, or point to any trend at all. The values spanned from zero to 4+ on the EB scale.

And the PI was one of the more recognized of the world's hyperbaric scientists (on more than one occasion, our own @Dr Simon Mitchell has cited him), so it's not very likely that there was any fundamental flaw in the experimental design. But the take-home message was pretty clearly that "divers are very different".

 

[DawgFan]

Let's say that the oft-quoted statistic that half of DCS cases occur to people diving within limits is accurate. People seem to think this indicates that diving within limits makes no difference, but that difference is enormous. At least 95%, possibly more than 99%, of all divers dive within the limits of whatever decompression system they are using, meaning that this huge percentage is providing only half of the DCS cases, while the tiny group that is violating those limits is providing the other half. It also means that the statistical likelihood of getting DCS, which is incredibly small, is at least half of that small percentage if you stay within those limits. If the percentage of DCS cases per dive is only 0.02%, as I have heard many times, that means people diving within their limits only have a rate of 0.01%. That sounds small to people who do not understand math, but it is actually an enormous difference.[/QUOTE

Let's say that the oft-quoted statistic that half of DCS cases occur to people diving within limits is accurate. People seem to think this indicates that diving within limits makes no difference, but that difference is enormous. At least 95%, possibly more than 99%, of all divers dive within the limits of whatever decompression system they are using, meaning that this huge percentage is providing only half of the DCS cases, while the tiny group that is violating those limits is providing the other half. It also means that the statistical likelihood of getting DCS, which is incredibly small, is at least half of that small percentage if you stay within those limits. If the percentage of DCS cases per dive is only 0.02%, as I have heard many times, that means people diving within their limits only have a rate of 0.01%. That sounds small to people who do not understand math, but it is actually an enormous difference.

At the risk of being a person who doesn't understand the math, I'm curious if I'm following the math. Using the estimated 95% of divers within the limits, 50% of DCS cases within the limits and 50% outside limits and DCS 1 out of 2000 dives (recognize this is debatable). In 4000 dives there would be 2 cases of DCS with one being from a diver within the limits and one a diver outside:

4000 dives * 95% = 3800 dives in the limits and 1 case of DCS so 1 / 3800 = .026% chance of DCS for divers in the limits

4000 dives * 5% = 200 dives outside the limits and 1 case of DCS so 1/200 = .500% chase for divers outside the limits.

Much better to have a .026% chance than .5% whereas the statement that 50% of DCS cases come from divers following the limits fails to capture this point. Am I following the math?

First time poster so please be gentle if I'm off. I've been reading the board for awhile and really appreciate the effort you and many others put in to it.

 

[KenGordon]

The other scenario is they don't take care of themselves, run aggressive profiles and are at a higher risk for DCS.

Why would they do that? They don’t know which group they are in.

My point is that it doesn't matter whether or not it is inevitable that a particular person gets bent in an infinite number of dives if you do not know if you are the particular person. I think we know that in the population as a whole some people will get bent. Maybe not every one, but some. So a random new diver ought to assume that will be him and dive accordingly as there is no way to know.

 

[tursiops]

Why would they do that? They don’t know which group they are in.

My point is that it doesn't matter whether or not it is inevitable that a particular person gets bent in an infinite number of dives if you do not know if you are the particular person. I think we know that in the population as a whole some people will get bent. Maybe not every one, but some. So a random new diver ought to assume that will be him and dive accordingly as there is no way to know.

You trying to make it too black and white. The argument is just to lower your risk, whatever it is, and not assume fatalistically that your number will come up sooner or later. Do what you can to make it later rather than sooner.

 

[Mark IV]

I used to stress to students that DCS was a very inexact "science", and the act of simply being a tourist, on vacation, in an alien environment, right off the bat gave them several negative strikes that enhanced their statistical likelyhood of having an issue ( i.e. probably not a fitness buff, unaccustomed to the hot,sweaty environment, late nights and lack of sleep, poor diet and daily/heavy alcohol consumption, etc.).
All this is why I always stressed a full 5-min safety stop, and enforced them on any dive I was in charge of. You can't be extra-cautious......afterwards.

 

[dmaziuk]

Just this morning we were talking about debugging program runs driven by random numbers, large data sets and so forth, and this came up: Variance reduction - Wikipedia

One of your variables is the individual diver. Another one is the number of dives. The other is kinds of dives and profiles and so on. And then your diver changes (grows older) as their dive count changes, and the rate of changes is different for everyone. So good luck trying to find practical meaning in the numbers.

 

[MargaritaMike]

I was just beginning to breathe easier, but now I can't. Based on all of these calculations flying around I figured that if I could document that 4 cases of DCS has hit an older, overweight diver that I was safe for another several hundred dives - which may be the extent of my diving career. Then I got to thinking about that number and its calculations. Over what period of time are is the test sample completed? Is it at a given moment? Is it hourly? Is it daily? Is it per location? Is it per dive operator? Is it by age? weight? height? Blood Pressure (average)? A specific combination of A,W,H,BP? Type of Gas used? Gosh, I could go on and on. Sooooo, the bottom line is dive every dive with the knowledge that THIS is the dive I will get bent and therefore adjust my profile some to maybe avoid it. After the dive, if not bent, I can say with all sincerity that I cheated death again (a phrase that we said as we were walking to flight control after landing our fighter aboard the carrier). While DCS is not always fatal, it CAN be fatal, therefore, not to be taken lightly - by ANY diver on ANY dive.

Cheers - M²

btw - as a mathematician I LOVE numbers.

 

[Bob DBF]

One of your variables is the individual diver. Another one is the number of dives. The other is kinds of dives and profiles and so on. And then your diver changes (grows older) as their dive count changes, and the rate of changes is different for everyone. So good luck trying to find practical meaning in the numbers.

The good news is if you have been diving a long time with a lot of dives, you are in a position to make good decisions on the the amount of risk you will take. Personally, I won't make the dives I would years ago. Hell, in the last 10 years or so I got a computer, ascend at 30'/min, make a safety stop, and don't go out in bad weather as much. That's as practical a use of the numbers as I can manage now.


Bob