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Gym climbing difficult tonight 9-Sep-2005 At 3:23:44 PM chris

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Somebody wrote that adding an extra screwgate is adding a point of failure.
I would also suggest that the knot tying is a point of failure.... sure the knot is foolproof, but unfortunately the knot-tier is not!! (too many not/knots there).

I did a course a couple of years ago which was about error management in clinical medicine (and air travel as well). One of the major messages of the course was that human error is inevitable. Human error is the most important fault in any system to be accounted for.
When done correctly, the reality is that both biner/biner systems and knot/biner attachments are absolutely bomber, and you could hang a whole lot more than the average 80kg bumbly from them.
The problem is of course the human factor.

I would assume the backup to the human component in the knot/screwgate system is the screwgate.
The backup in the biner/biner system is the second screwgate.
Human error is hopefully reduced for the biner/biner method by only having to remember one way to attach to the rope (ie using a screwgate).
Using a screwgate is usually easier for a beginner than tying a knot, because most people have used a screw/thread in the past (eg to put lids on a jam jar). Conversely, tying a figure of eight is a "new engram", and takes a little while for people to learn.

The other way to reduce human error is to introduce checks and balances in a system. The rockclimbing check is for your belayer to check your tie-in. It is fairly basic to check that the screwgates are clipped and screwed up. Unfortunately for the knot/biner system, this assumes a high skill level of the belayer... ie the belayer must allways be able to recognise a properly tied figure of eight.
The result is that the biner/biner system minimizes human error by making the system easy to use, easy to check, and easy to teach. The biner/knot system is good for experienced climbers, but for beginners, is harder to use, harder to check, and harder to teach.

One of the other problems in error management is that experienced people become overconfident.... who has never tied a figure of eight incorrectly, and set off to climb only to see that they had a granny knot hanging off their harness? Conversely, I have often forgotten to screw up my screw gates, so this system is also prone to "overconfidence error".

Perhaps the safest method is to use two twist-lock biners, which are basically failsafe, as they cannot be attached to the rope without being locked. I suppose the problem with them is they seem to wear out quickly.

Of course it's all academic for outdoor climbing, because a higher level of skill is assumed, and for some reason we have settled for everybody just being tied into the rope. What this means, I suppose, is that all of the gym systems in reality are "oversafe", because you only really need one tie-in point.
The second attachment is just an example of a good check and balance built into any safety system.

Hope I haven't bored everyone to death with this "error analysis" of rope attachment.