Is anecdotal evidence dangerous?

[Cacia]

to GroupThink. . .

excellent, your links always worth the trip, Kev. "reality testing" gets diluted for sure...
great remedy list! That deserves it's own thread. Give examples, etc would be exceptionally useful exercise. Do you DIR people use that in your training?

 

[captain]

He also said:


Take the Navy Dive Tables: they are a SWAG and should never be considered to be foolproof.

An which tables are fool proof?

 

[Thalassamania]

An which tables are fool proof?

Nothing is foolproof, 'cause them fools is so damn ingenious.

 

[Kevrumbo]

excellent, your links always worth the trip, Kev. "reality testing" gets diluted for sure...great remedy list! That deserves it's own thread. Give examples, etc would be exceptionally useful exercise. Do you DIR people use that in your training?

Actually my dear, that was my kudos to the Mods for retaining the Open DIR Group, while spinning-off the DIR Practitioner's Sub-Group

 

[captain]

You're not telling me anything new, but that's all we had from 1956 until the 1980s.

But here's the interesting thing, the recreational community had a horrible record diving U.S. Navy tables while the science community had an excellent one. There have two explanations proposed, and maybe it was a combo:

1. recreational divers did a really poor job of controlling there ascent rates. There were a couple of studies (Egstrom at UCLA, I think) where recreational divers ascent rates were measured whilst the divers were unaware and rates were typically between 100 and 120 fpm! And don't forget that in the early days a diver learned that he or she was low on air by running out. It took discipline and comfort to make a slow ascent whilst breathing through a straw.
2. the U.S. Navy tables were too hard for most people to learn (until NAUI reformatted them circularly) and have way to many "special rules." People just couldn't work them routinely, and this included instructors who rather than demonstrate their stupidity sluffed over them with a lot of hand waving.

I'm interested in anyone's thoughts on the subject.

Thinking back to the may years I dived the Navy tables I think the ascent rate may be more of a factor than knowledge in how to use the tables. At least for me I had a good understanding of the tables but I can't remember paying too much attention to ascent rate other than the old " don't exceed your smallest bubbles" routine and this was in pre safety stop days. I did make this far without ever having to take a chamber ride so maybe I was doing something right after all like not pushing the limits

 

[Thalassamania]

... I can't remember paying too much attention to ascent rate other than the old " don't exceed your smallest bubbles" routine and this was in pre safety stop days. I did make this far without ever having to take a chamber ride so maybe I was doing something right after all like not pushing the limits

Perhaps you're right ... we were all taught to ascent by our watch and depth gauge (and 60 fpm is 1fps, which is easy). 5 feet on your gauge, then 5 seconds on your watch, repeat to the surface.

 

[boulderjohn]

You really can't lump all anecdotal evidence into the same pot and judge them as if they were all equal. When someone says he dove to 150 feet on air and came up without a safety stop, therefore all such dives are safe, we cannot trust that because one incident does not give us enough information to make such a judgment.

On the other hand, I once read in a SB post that--and this is not an exaggeration-- all alternate air sources worn in the traditional triangle on the chest method come loose on all dives, drag in the silt, and tear their diaphragms, then a single anecdotal experience where that is not true refutes the absoluteness of that claim. If you can anectodally say that in your own diving experience you have never even heard of a case of that happening, then you can be reasonably certain that the claim is not even true as an example of acceptable hyperbole.

Anecdotes can effectively demonstrate that a rule is not absolutely true. A dive to 150 feet without a safety stop does in fact prove that such dives are not universally harmful. It does not, however, prove that they are universally safe.

 

[Geoff_H]

When anecdotal data can be tested, verified and a definite and predictable result.

Tables are anecdotal evidence according to your test as you can dive within the limits of tables and still get bent. Maybe this will only happen to 1 in 1,000 or 1 in 10,000 but it will happen.

 

[fweber]

He also said:


Take the Navy Dive Tables: they are a SWAG and should never be considered to be foolproof.

Whenever I plug USN data into V-Planner, I get lots longer times than the USN tables require. This makes me truly wonder about the USN tables and not be inclined to try them at all.

That's not telling you anything new, Thal, I am sure.

Thal, Gene or anyone else knowledgable with the history and development of tables will hopefully correct me if I'm wrong. But, I believe the USN may have conducted or funded more scientific study of decompression than any other singular organization(based on nothing but a hunch). Their tables are based on thousands of real, objectively studied dives, conducted I'm sure to the best of scientific method. Derived from these studies is a set of tables based on the real world probability of those divers getting bent. However, their subjects and the average recreational diver don't align. We're not all young and extremely fit.

Nereas, as you've suggested, I would contended that you are realying more on anecdotal evidence than scientific method. Vplanner, which I use and trust, is a theoretical model, trying to predict the possible outcome. And, while many dives have been safely conducted using it, I don't believe the actual study of those dives comes anywhere near what has been done with the USN tables. Furthermore, the body of dives that have been studied are those submitted by divers themselves, hardly a poster child for objective scientific method. Is Vplanner based on science? Yes, theoretical body chemistry and physiology. Has it been well tested by objective scientific method? I'm sure Ross could inform us to what extent. But, probably not as thoroughly as the USN tables. However, we hear good things about the outcome of Vplanner(and other newer softwares) dives and based on those anecdotes choose to use it.

The USN tables are probabalistic in their nature. Take a bunch of young men, put them at depth for a time, surface. See how many get bent, determine the standard deviation, adjust to provide an acceptable probability of DCS(very low). They're based on thousands of objective dives. And arguably are supported by a less anecdotal method.

I'm not pro USN tables(although we do carry a foldable set in a pocket as a SHF contingency), nor am I opposed to Vplanner(on the contrary I use it and think it's one of the better planning programs, with one of the better user interfaces). I'm just trying to point out that the tendency to dive in what might be considered a more conservative manner, longer deco curves and lower END, may in fact be based on more anecdotal evidence than the the conventional methods. That was certainly the way that nitrox came to prominence and is the by and large the way helium is working it's way into sport or recreational diving.

So, Anecdotal evidence is neither good nor bad. It just has to be treated accordingly.

 

[The Chairman]

An which tables are fool proof?

None... all the current tables (with the exception of the newer NAUI RGBM tables) are clones of the Navy tables with some fudge factors applied to make them safer. Navy tables were derived from fit teenagers, not dumpy 45 YO divers. When the premise is anecdotal, then the result must be the same.

Clint Eastwood, as Dirty Harry, once opined: "A man has to know his limitations." This applies to SWAGs such as dive tables AND computers. In statics, after they teach you all about structural analysis with moments of inertia and such and then they spring those "statically indeterminant structures" on you. Way too many variables to KNOW how the structure will stand (or fall). Quite often, they approach the design of similar structures to try and deduce the design parameters to keep them standing.

The same is true for on/off gassing. We are confident that certain variables affect our body's ability to process gas, but we have yet to quantify them. So in fact, we use quite a number of anecdotal procedures to add a safety buffer to our table output.