Doc,
Just to make sure that I understand you point, you are saying that there is indeed benefit from using nitrox but diving air NDL limits. You are also saying that this benefit is based on "logic" and is not proven from experimental data but it is "thought" that one "should" be safer in terms of reducing the chances of DCS but no guarantees.
Well, I just dig out earwax for a living - I'm not a DAN dive medicine guru, and I wouldn't really make such definitive statements out of hand..!
However, I do know a good bit about research protocols and statistics used to draw supported conclusions. It is very important that all divers understand the nature of the science that we are using to keep us safe and healthy. Also, you should remember than there are NO guarantees at all in this sport - that is what an "undeserved" hit means. Tables are great, but there is a bell curve for everything.
In order to show a difference (between nitrox and air, or between two drugs, or between surgery and medical therapy, or whatever), you need to have valid evidence. However, there are different degrees of
quality of evidence - ranging from the gold standard of a double-blinded, controlled, clinical trial down to mere expert opinion (which is frequently evoked as gospel, but not as strong from a scientific point of view).
The strongest type of evidence for a reduced undeserved DCI hit rate with nitrox vs. air when diving air tables would be generated by a study that might never be done - a double blinded trial. You would take N number of divers and randomize them into air divers and nitrox divers. You would then have them do identical profiles (or matched cohorts with different profiles, but with one air diver for every nitrox diver for a given profile). You would then have observers determine whether or not they had DCI without knowing which gas they were diving, and see if you could get data to support your hypothesis (reduced DCI with nitrox).
So what is N? That would depend, mainly on how common undeserved DCI was. If it turns out it happens once in 20 dives with nitrox, and once in 10 dives with air, you wouldn't need that big a study group. If it happens once in 100,000 dives with nitrox, and once in 95,000 dives with air, you would need a huge study group to pick up that small difference.
Also, there is the question of clinical significance even if you have demonstrated statistical significance. Statistics are funny things, as are the way that human beings use them. Look at the recent cell phone - brain tumor thing. Suppose that using a cell phone raises your risk of getting a brain tumor, but by less than flying across country does (increased exposure to cosmic radiation). You would have to determine if you personally are willing to give up using the phone in order to get that minuscule advantage.
Same with nitrox, if you are using it to avoid undeserved DCI, you need to know exactly how big an improvement in safety you can actually expect. If it turns out that you are more likely to be killed in a car accident while driving to your nitrox class (possible if the added safety benefit is low enough), then it's not worth it! Unfortunately, that number is hard to come by (as discussed above).
Finally, some things you just have to accept without having valid statistics. No one has done a double blinded study to prove the benefits of doing a tracheotomy in cases of acute complete airway obstruction that can't be fixed by other methods, so I guess we just do them because logic dictates that if you don't, the patient will die. You aren't going to get anyone to volunteer to be in the control arm of that study...!
