Is anecdotal evidence dangerous?

[Thalassamania]

I will agree with you: they are not in fact "anecdotes". They are comparing apples to oranges, or goats to humans as it were.

One of the first evidences of "bubbles" as being a part of DCS was a researcher (Haldane, I believe) seeing a bubble in a serpents eye that he had just decompressed. Is this an anecdote, a data point or both? As was pointed out, any dive whether it resulted in an incidence of DCS or not, is merely an anecdote. Combine a number of these together and we have a "study". .

Anecdotes lead to hypotheses, hypotheses lead to experiments, experiments lead to knowledge. That’s the system.

The final product was not based on goat testing, goats were used in initial experimentation for a very specific reason. Do you know why goats were used in the early experiments?

The real problem is the extrapolation of these data points.

Actually the real problem is a public is woefully unconversant with scientific method and terminology.

For example:

One person has claimed that they bounced to 165 fsw and had no effects. Can we extrapolate that data to everyone?

Actually I not only make that statement (160 for 5 I think was the schedule) but will expand on it to say that I’ve done it more than once and I suspect it has been done quite literally thousands of times, by many different people since the U.S. Navy Tables came out. The 160 for 5 schedule was never one that was felt to be suspect.

Now lets deal with the next piece of misinformation that your spreading:

What if we had a thousand young and fit divers bounce to 165 fsw? Would no DCS provide us incontrovertible proof that a dumpy 45 YO would have no problem bouncing to 45? No, now it becomes a SWAG.

Contrary to the urban myth that the U.S. Navy tables were developed for, and exclusively tested on, young peak-fitness seamen, I have it from Bob Workman himself that at least one of the test groups was composed almost entirely of “middle-aged, overweight, hard drinking, cigar-chomping, U.S. Navy Chiefs.

How about if the water is 20 degrees colder than the sample? What if... (put your own variable here)???

The real problem is not the anecdote, but the fallacious conclusion drawn from the data at hand. If I said I did a half hour dive to 60 foot and did not suffer any symptoms of DCS, this would not only fall within the traditional data (as defined by the tables) but it would also be an anecdote for me to go on. It's the anecdotes that seem to DEFY our understanding of decompression physiology that disturb us the most.

Once the tables are developed from a hypothesis, tested experimentally with animals and tweaked, test experimentally with humans and tweaked and then tweaked on a regular basis as a result of operational experience, they’re not anecdotal. What then is anecdotal are reports of people who got bent diving inside of the tables. As has been observed before, many anecdotal cases of “undeserved” bends likely result from bad ascent procedures.

 

[The Chairman]

As has been observed before, many anecdotal cases of “undeserved” bends likely result from bad ascent procedures.

That's a SWAG if I ever heard one. In fact you made several of them in your post.

Tables are merely tools. Learn their limitations so they don't bite you.

"Undeserved hits" is a misnomer. "Not understood hits" is a better way of describing them, and they may in fact have an ascent that caused the issue, but was still within prescribed limits.

 

[fweber]

Thal, give it up. On this subject Netdoc is acting like nothing more than a come troll. What's worse is, he does understand, but is choosing to try to wind you up for some reason.

 

[The Chairman]

Thal, give it up. On this subject Netdoc is acting like nothing more than a come troll. What's worse is, he does understand, but is choosing to try to wind you up for some reason.

I resent this. I have not called anyone a name in here. There is no need to resort to such when there is a disagreement.

 

[Rockhound76]

Anecdotes lead to hypotheses, hypotheses lead to experiments, experiments lead to knowledge. That’s the system.

Thanks. I was about to post exactly the same thing. Anecdotes are often the foundation for hypothesis and a hypothesis is the beginning of an experimental, sometimes iterative, process.

You also sed:

"Now lets deal with the next piece of misinformation that your spreading:
Contrary to the urban myth that the U.S. Navy tables were developed for, and exclusively tested on, young peak-fitness seamen, I have it from Bob Workman himself that at least one of the test groups was composed almost entirely of “middle-aged, overweight, hard drinking, cigar-chomping, U.S. Navy Chiefs."

Great info and news to me. I was taught as you stated, that USN Standard Tables were derived from testing on "peak fitness USN divers". Additionally, I was told that a certain "statistically significant" incidence of bends was built into the tables, as this was acceptable to the USN.

I pretty much assumed the second part was bull, but believed the first. In this case, I'll accept your anecdotal second-hand information as it fits my preconcieved notions. Why? Because I can and because it provides some assurance that the USN wasn't stupid and I won't die on those tables. Better yet, I used them for years and didn't die so it confirms my experience. See? Some anecdotes are okay, if we recognize them for what they represent.

Thanks.

 

[Teamcasa]

…As to anecdotes. The OP seems to believe that they are, in fact, dangerous. While I think several of us believe them to be nothing more than additional data points, that require more careful consideration before use in a decision making process.

Let’s try not to put words in my mouth. I will say that when a diver or anyone else for that matter, relies solely on anecdotal evidence or testimonials for their safety when doing some activity outside of tested norms, they may have and should expect some problems.

Very frequently, strong anecdotal evidence is what leads to the adoption by a larger user base and further research that can provide factual support for a particular idea or practice.

On this I do agree.

Anecdotal evidence is neither safe nor dangerous, it's someone's tale, take it as such, consider who is telling it, and what their experience level is or has been. I tend to not follow the advice of the extremely youthful, or those who are obviously following a path through life very different from the one I wish to travel. But, that doesn't mean I don't listen to it............

I am also in your camp on this. I too listen to many opposing points of view, then decide for myself.

 

[fweber]

I resent this. I have not called anyone a name in here. There is no need to resort to such when there is a disagreement.

I'm sorry if you resent the label I see you representing in this 'disagreement". SWAG seems to be your pet acronym of the day. While I could respect your choice, if it were to start a new thread about the pattern of logic, reasoning and study that lead to something being considered "fact" as opposed to "I guess". I don't have to respect your repeated attempts to distract this thread from the discussion of the realative safety or danger of anecdotal evidence. Even if Thal, I, or anyone else for that matter don't site a reference for each and every contention, it does not mean those contentions are nothing more than "Wild *** Guesses". To label every thought that way is potentially more insulting than to have simple called names, as you're accusing everyone of not exercising either logic or reasoning. The only other option is to simply transcribe the entire Rubicon Archive in this thread. But,.........you'll just contend that they were all guessing.

 

[The Chairman]

But,.........you'll just contend that they were all guessing.

This is a complete distortion of what I have been contending. Thal took exception of me calling the tables a SWAG. They are nothing more than that. They do not MEASURE a single tissue group. They have NO IDEA of my age, gender, body type as well as a number of other variables. To this end, I have shown WHY I believe this to be true. Also, I specifically use the word SWAG when I teach tables. It's not a word du 'jour for me.

I did point out a few declarations that I consider to be more than SWAGs: like the use of "horrible" in conjunction with sport diving's record on DCS. There has been no substantiation of the record being "horrible".

The problem with these types of discussions is that quite often the arguments rely on the fallacy of fear, of poisoning the well, ad hominems or hasty conclusions. The easy way out is to label the person pointing these fallacies out as a troll. That is a red herring AND poisoning the well.

 

[fweber]

I have read so many posts that use only anecdotal evidence to support a particular point of view. To me this is down right dangerous in some cases. When it comes to diving, I see this as very troubling. How many times have we all read “ I’ve been to 160’ fsw on air and did fine” or “I came up from 100’ and did not do a safety stop” or “I don’t get narcked at 120’ fsw”. This sends a message that if I did it, you should be fine so go ahead and try it! I am always dubious when the only evidence is anecdotal.

I believe in the premise that science trumps anecdotal evidence every time. This not to say some have in fact done some of these things but is it safe for anyone, no. So when someone presents a situation where the only evidence is anecdotal, do more research before trusting the information.


.

Let’s try not to put words in my mouth. I will say that when a diver or anyone else for that matter, relies solely on anecdotal evidence or testimonials for their safety when doing some activity outside of tested norms, they may have and should expect some problems.



On this I do agree.



I am also in your camp on this. I too listen to many opposing points of view, then decide for myself.

I'm sorry. Upon rereading your first post I see clearly your careful use of the word "only" preceeding your concerns of anecdotal tales. On the point that it takes both anecdotal and factual data to make an informed decision we agree.

With regard to your examples, I believe you could have chosen better ones. Two out of three are in fact examples of someone relaying anecdotal information that confirms what has been supported by scientific study and real world practice. The third, regarding narcosis, is a highly subjective subject. For some one to say "not" should be difficult. But, for someone to say "still capable of performing satisfactorily" would not run counter to what is commonly believed by NOAA, USN, AAUS, to name just a few.

Of far more concern to me, is the anecdotal support and spread of things that lack supporting research as far as I'm aware(if there is some please point me in the correct direction) such as "on the fly deco" or "ratio deco", the mutliple different ways people decide to come up with their own deepstops, and the proliferation of shallow, rich, helium mixtures. I know that the principles of these are based on scientific theories, but is there any objective, supporting study, following scientific method, that confirm the realative safety of each? Someone has to be the "early adoptor" for these, just as people were for nitrox and the use of helium in general. But there are a lot of people with no more or less experience than I spreading their anecdotes as they relate to these subjects. And then accusing others of being UNSAFE for not following suit!

 

[Thalassamania]

"Undeserved hits" is a misnomer. "Not understood hits" is a better way of describing them, and they may in fact have an ascent that caused the issue, but was still within prescribed limits.

No, what I was really saying was that the recreational divers who took these "Undeserved hits" more than likely “Deserved” them because of poor ascent procedures.

Thal, give it up. On this subject Netdoc is acting like nothing more than a come troll. What's worse is, he does understand, but is choosing to try to wind you up for some reason.

I’m glad that you see it too. Pete took a dislike to me at some point as a result of what he feels is my condescending attitude (and its true that I don’t suffer fools gladly) this seems to result in Pete twisting himself into highly illogical positions in an endless and really boring game of “Gottcha!” It’s a cross I have to bear if I want to play, and I’m resigned to that.

… Additionally, I was told that a certain "statistically significant" incidence of bends was built into the tables, as this was acceptable to the USN..

Please allow me to let my colleague “Harris” Taylor deal with your second misconception: The US Navy Dive Tables 5% Failure Rate Myth.