Discussion of the statistics of the NEDU study on the redistribution of decompression stop time from

[Dr Simon Mitchell]

Simon, The nasty accusation above is your own...not mine. Please stop trying to project it onto me.

The following are comments you have made about this issue in various posts so far.

"It's my opinion the test stopped early to salvage what they could from a expensive test procedure that was about to be scrapped. This story line we hear today about excess injury cancelling the test, is just not true".

"Like I said, I think they salvaged this test from imminent rejection, and the reason quoted above is the excuse".

"As I said before, I am inclined to think that the mid point analysis was more of an excuse to stop the test that has gone off the rails, than anything else. Obviously It makes sense to stop the test half way and salvage what you can, and that is what I think really happened".

"The elaborate mid point test limits, were only added to the 2011 report. So which version do we believe? The original technical version, or the re-written for public consumption version?"

"Yes it does say that.... written after the test was done. We have no guarantee that was a decision point or value before the test".

"Where did the extra 2011 rule come from? It's not in the 2008 reports. This new 2011 rule and condition conveniently fits into the events as they happened".

"You can see why we should be skeptical of new reasons added to old reports".

There is no possible way to interpret this other than, on multiple occasions in this thread, you have explicitly accused the authors of the NEDU study of fabricating a new study end point so that they could "salvage" something, and then claiming that it was always one of the end points thereby misleading the community. In other words, you are accusing them of scientific fraud.

If this were not so serious it would be funny, because your entire thesis is based on your inability to comprehend the issue of the stopping rules (your belief that the study was "headed for rejection"), and you persist with this ludicrous crusade despite being told, time after time (including by the study's lead author), that you have misinterpreted this.

The test did stop half way.... because it was headed for rejection.... according to the rules at the time. Naturally it had to be stopped and salvaged, for any number of reasons. Its trend line and direction was obvious to all involved.

When are you going to stop making a fool of yourself over this?

Simon M

 

[Dave Bevan]

Thank for your input Dr Simon Mitchell and also your continued patience.

 

[rossh]

How many times do you have to be told that there was no impending rejection:

Simon M

The impending rejection......

Was the A1 going to make it to the finish line?

.

 

[Dan_P]

I find this super interesting, and the article was both stimulating and illuminating, so thanks for the contribution @Gozo Diver.

I personally have reservations about the extent to which I can utilize the findings of the NEDU-trial in practice (though I'll readily acknowledge "some", whatever one might perceive that to actually mean). My main concerns relate to thermal state and gas choice - I am curious about the basis for determining that those two factors were isolated and accounted for - but this read was an unexpected pleasure with a different angle on things.


Best Regards,

Dan

 

[David Doolette]

I do not want to hi-jack this thread further away from the very clear and accurate post on the limitations of the NEDU deep stops trial (thank you Gozo Diver), and certainly do not want to re-hash all that was discussed in the several other threads, including a 1000+ post thread on this forum, but since Ross is essentially accusing my co-authors and I of scientific fraud, I just want to correct the several falsehoods on which Ross is basing that argument.

Ross is trying on two different, mutually exclusive, narratives about the midpoint analysis that resulted in the trial ending. The first of his narratives is that we conducted the mid-point analysis during the trial to “salvage” the trial because the A1 schedule was nearing the sequential trial stop-low criteria and the trial was at risk of terminating (posts 17 and 54) – this would have had to have been in 2005-2006 while the diving was underway. The second of his narratives is that we added the mid-point analysis between the 2008 UHMS workshop proceeding which he misrepresents as the "original technical version" and the 2011 NEDU Technical Report which he misrepresents as "re-written for public consumption version" (post 41 and 51).

To address the last bit of this fabrication first. The definitive report is the 2011 NEDU Technical Report (Doolette, Gerth, Gault, Redistribution of decompression stop time from shallow to deep stops increases the incidence of decompression sickness. Panama City (FL): Navy Experimental Diving Unit; 2011 Jul. NEDU TR 11-06). Readers who are familiar with science will know that a conference proceeding is often a preliminary report, as was the case when we invited to present our findings at the 2008 UHMS workshop (Gerth, Doolette, Gault Deep stops and their efficacy in decompression: U.S. Navy research. In Bennett, Wienke, Mitchell eds. UHMS; 2009. P. 165-85) and 2008 DAN conference (Gerth, Doolette, Gault Deep stops and their efficacy in decompression. In Vann, Mitchell, Denoble, Anthony eds. Technical diving conference proceedings. DAN 2009. P138-56). Ross may not know how science works, but the rest of his narratives are willful misrepresentation.

Having claimed to have read the UHMS Workshop, he must know that it presents the methods and findings of the NEDU study very briefly, but mentions the midpoint analysis (On page 178 of the UHMS workshop proceedings. Also on page 152 of the DAN conference proceedings). So Ross's claim that the midpoint analysis was added after these preliminary reports for "public consumption" is false.

Ross's other narrative is that we added the midpoint analysis to "salvage" the trial because the A1 schedule was nearing the sequential trial stop-low criteria and the trial was at risk of terminating. This is incorrect for two reasons, both of which have been explained to Ross in several forum threads. First, as has pointed out in several posts on this thread, one schedule reaching a stop-low criterion would not have terminated the trial. Second, as I have pointed out to Ross in other threads, he is misinterpreting what is admittedly a poor illustration of the sequential trial envelope in the figure he shows (post 17 and 54). In that figure, the envelope is drawn such that the schedule has to CROSS the envelope to hit a stop criterion (in the NEDU Technical the envelope is redrawn such that the trial has to TOUCH the envelope, because that is more intuitive). At the mid-point analysis, the A1 schedule had 3 DCS in 192 man dives (3/192). The stop-low criteria in this vicinity based on 95% confidence the risk of DCS was less than 3% was 3 DCS in 256 man dives (3/256). In the figure Ross shows, the A1 schedule could have touched the envelope line in just a few more DCS-free dives, but the envelope is horizontal at that point, and the stop-low would not occur until the trial CROSSED the envelope by emerging on the other end of that horizontal segment with 3/256. In other words, there needed to be another 64 DCS-free dives before we reached a stop-low for the A1 schedule.

The midpoint analysis was part of the trial design, it was conducted at the midpoint of the trial (in 2006), the trial was nowhere near another termination criteria, and the midpoint analysis was reported in the preliminary conference proceedings in 2008 and 2009, and in the final report in 2011.

David Doolette

 

[RyanT]

To address the last bit of this fabrication first. The definitive report is the 2011 NEDU Technical Report (Doolette, Gerth, Gault, Redistribution of decompression stop time from shallow to deep stops increases the incidence of decompression sickness. Panama City (FL): Navy Experimental Diving Unit; 2011 Jul. NEDU TR 11-06). Readers who are familiar with science will know that a conference proceeding is often a preliminary report, as was the case when we invited to present our findings at the 20008 UHMS workshop (Gerth, Doolette, Gault Deep stops and their efficacy in decompression: U.S. Navy research. In Bennett, Wienke, Mitchell eds. UHMS; 2009. P. 165-85) and 2008 DAN conference (Gerth, Doolette, Gault Deep stops and their efficacy in decompression. In Vann, Mitchell, Denoble, Anthony eds. Technical diving conference proceedings. DAN 2009. P138-56). Ross may not know how science works,

Just a +1 for David's comment on this. Indeed, conference proceedings are always truncated presentations of a full study. Sometimes, they are even preliminary as studies are continued after a conference. Thus, pointing to one of these proceedings as failing to be fully representative of the details of a study just doesn't hold water.

 

[Dr Simon Mitchell]

My main concerns relate to thermal state and gas choice - I am curious about the basis for determining that those two factors were isolated and accounted for

Dan, if you are really interested in this there is extensive discussion on it elsewhere, and no need to rehash it here. I would start with this, and then read the whole thread, and the one that runs on from it.

Simon M

 

[Dan_P]

Hi Simon,

Indeed, I am.
I didn't, and don't, mean for this to be any real argument, but rather raising genuine questions I'm actively trying to answer at present.

On a quick sidenote, the string linked to above, was infinitely more entertaining to read, than it was to re-read.
In case my Rosetta Stone is in there, my attention must have failed at approximately the same time of processing on both occasions.

While I agree that we shouldn't derail this conversation on statistics (which is certainly an interesting avenue of it's own), I will say in response that I do understand the role of thermal stress as an aggregator, and I do appreciate it's place in the relevant setting.
I also am aware that the water and exposure protection was identical across the two groups of trialists - what I fail to understand is how gradual reduction in ability to off-gas due to (some level of) hypothermia would not be of particular disadvantage to an ascend procedure that is comparatively deeper as the off-gassing ability diminishes, all else being equal.
I am looking for works done or insights that would support any conclusion on that question.

I am also aware that using air for the trial was a carefully chosen and aptly motivated direction to take - I'm just preoccupied with three legs of consequence that choice may (or may not) have had - or rather, how/if we know it didn't;

1) If the - presumably significant - CO2 retention from the bottom section would imply an extended residual CO2-load during deco stops at greater gas pressure as opposed to lessor gas pressure.
(i.e. did residual CO2 off-gas quicker on the shallower stops, and could it be relevant?).

2) If there is any difference across how He and N2 diffuses relative to depth
(i.e. "is a given deep stop more/less effective when breathing one gas compared to another?").

3) If the use of air in and by it's own had an impact due to it's kinetical properties or poor general properties as a deco gas, separate of the above.

Further, might not the tipping point between any (theoretical, if you will) benefits of a deep stop versus slow-tissue on-gassing during a too deep stop be pushed by the choice of air for both bottom gas and deco gas?

"If decompression stops are conducted using a breathing mixture with a low inert gas fraction, then, of course, there is less gas uptake into the relatively slow compartments. The effect of this is to increase the depth at which stops become “bad” deep stops.

David Doolette"

Admittedly, my questions are hypothetical (hence their interrogative state), but this is not to say that I don't understand why the decision was made to use thermal stress and air during the trial.

My point is that appropriateness of lacking statistical significance of a two-sided Fischer's is not at the top of the questions I'm trying to negotiate.


Best Regards,

Dan

 

[tridacna]

@Dr Simon Mitchell

Animadvertisine, ubicumque stes, fumum recta in faciem ferri?

I implore you not to sink to their level and respond to the trolls. The original post was interesting and useful until Ross and Dan realised that this was yet another opportunity to bash you and David. I don't think that their nefarious questions or accusations deserve a response. Doing so implies their dubious legitimacy.

 

[Dr Simon Mitchell]

Hello Dan,

Hi Simon,
My point is that appropriateness of lacking statistical significance of a two-sided Fischer's is not at the top of the questions I'm trying to negotiate.

Well, perhaps this thread which is about statistical interpretation is not the place to be negotiating your questions in.

There are good answers to all of them which you can find elsewhere on line. If you can't find them, either start a new thread or pm me if you want to discuss it. The problem with doing it here (or starting a new thread for that matter) is that it will inevitably attract Ross's attention, and he clearly sees no problem with attempting to mislead readers by rehashing his uninformed incorrect interpretations even though they have been debunked over and over and over. This will force me into having to do it all again, as I have had to here, and I have no appetite or time for it, and neither does Prof Doolette.

Simon M