I've not read all the replies, but I've skimmed most of them. I probably am not saying anything new, since I've not seen anything new since the very first day of the TDS version of this thread... but since Andy seems to think he can repeat himself ad infinitum, then I suppose I can post again. [thanks to GDI for reopening].
First off, let's look at statistics. Statistics can be spun anyway you please... a practice very commonly seen in marketing as well as some lawyers too. For example, I once corrected my marketing director when she used the term "many" in a publication. "Laura, there's only two". Her response was that in marketing, you have 'none', you have 'one'. You have 'many' which is 'more than one, positive' and you have 'few', which is 'more than one, negative'.
If you want to use statistics for the argument, go right ahead, but you're not going to convince anyone because people will know it's bogus. If you'd like to actually have a valid argument, you need to use a scientific approach and must limit your sample set to remove the causative differences. In other words, if you are going to compare singles vs. doubles, you have to actually remove some cases from the sample space. You've already removed all but intro divers from your space.
The grand flaw in your argument is that you've constrained the 'singles' sample space to 'within limits', whereas you've not contrained the 'doubles' sample space accordingly. I'll accept that no intro diver has died on a single tank within training limits, it sounds plausible and I don't feel like doing the research to see if there are any that died of medical conditions. But constraining the doubles sample set similarly, I think you'll find that the statement holds true also. So we have zero and zero. With that you can continue your argument to make rediculous statements as well; like stating that it's safe to dive at the intro level with no fins and covered in entanglement hazards, because no intro diver diving within the limits of their training have died with no fins or entangled in the line. You can add anything you want to "no intro diver has died within training limits' as long as what you add doesn't expand the sample set.
Now if you want to throw out the training limits statement, then throw it out. If you want to state that no intro level cave diver has died on a single, then I could use your point to state that jumps and gaps are safe with an intro card, or diving with a stage and a single, providing your in a single tank, because there is no statistics that state otherwise. But if we drop the 'intro card', then guess what, we just found the P3 fatality of a cavern diver diving in single tanks with a stage. If we make it 'not full cave', then we just opened it up to all the OW divers that have been killed in caves.
So you've proven that no intro diver, diving within their limits on a single tank [1/3 of gas] has gotten themselves killed. Likewise no doubles diver, diving within their limits on a set of doubles [1/6 of their gas] has gotten themselves killed. So that statistic proves nothing.
So apples to apples, it comes down to what you are comfortable with. I personally feel for any overhead environment that the hassle of doubles with the added entanglement hazard/failure point of an isolator is worth the risk. Andy, if you feel that the chance of a burst disk or neck o-ring failure is miniscule to the point that it's not necessary to consider it... do you dive a straight bar or do you add the failure point of an isolator?
Enough with that part of things. Now a lot of the arguments seem to revolve around catastrophic gas loss as the reason for reserving 1/3 of your gas supply. While most things the extra 1/3 is reserved for are rare occurances, there are many that are more common then catastrophic gas loss.... like the added time it takes to exit in low visability. We all know from our training that in a no-flow system [e.g. walking a land line course] it's going to take more than twice the time to follow the course with your eyes closed. This, AFAIK, is to demonstrate that 1/3 is a -minimum- reserve, and one should reserve more gas if conditions warrent. For example, shallower low flow systems like peacock, where you can get much farther, one should consider reserving more than a 1/3d because you will be a bit farther in, and you'll need more gas to exit then if you were in a higher flow system.
On deeper low flow systems, like Little River was a couple weeks ago, time on exit in a problem situation becomes an issue. When I took cavern a few years ago one of the things I was taught was the addition of planning your NDL. 1/3 of gas, so you have 1/3 in reserve. 1/3 of NDL so you have 1/3 in reserve as well. While I've not asked a lot of folks, of those I've mentioned it to, I've never found anyone else that had ever even heard of the concept of 1/3s of your NDL to insure that you stay within your NDL limits. [I'm trying to stay off my 'lack of deco/adv-nitrox prereqs for full cave courses].
So if you are teaching 1/3 on singles, but not 1/3 on NDL, you now have to consider that should something happen, and the extra time it takes to egress cuts into your 1/3... the time also adds a need to cut into that 1/3 even more because you have deco to perform.
With the flow in Ginnie, where it takes significantly less time to egress than it does to enter, while you are reserving twice as much gas to exit as you needed to enter, you are actually reserving more like 4 times as much gas to exit as you need to exit [1200 in, 600 out, 1800 reserve]. In a low flow system, I think it's a damn good idea to do the same... for example if it's 1200 in and takes 1000 to get out, you've reserved a 3rd, but the practical reserve is only 1400, vs the 1800 at ginnie, to travel a much farther distance and longer time. Should you be diving in doubles, you now get that extra practical reserve.... 600 in, 500 out, leaving you a reserve of 2500 psi... which is ample to cover the added time.
So it seems to me that doubles -make sense- and should be encouraged for those with less experience in caves and are therefore more likely to run into situations where they use more gas to exit due to stress or visability or other factors.
The way I see this whole doubles restriction is quite simple. Some people [intro divers] are damaging [exceeding their limits] their furniture projects when they use screws. Because some of those people use a sledgehammer [doubles] to pound their screws in, let's limit them all to a smaller hammer [singles] so they can't do as much damage as they would with a sledgehammer. Of course those that know how to use a sledgehammer responsibly are now stuck using a small hammer to split wood.
Historical deaths are used to emphasize the importance of the 5 rules of accident analysis. I don't see why the intro diver accidents where they've exceeded their training can't be used to convince intro divers to stay within 1/6 of doubles...
If you want to limit penetration gas... do so. From what I understand NSS-CDS already has that restriction. If you want to limit penetration distance... do so. Something like (150-depth)*10 sounds like a decent restriction. (150-60)*10 makes for a 900 foot penetration limit for Peacock. (150-90)*10 makes for a 600 foot limit for Devils. But trying to control the bad apples by restricting the amount of emergency reserve makes as much sense as outlawing sledgehammers.
