In reply to CurlyStevo:
> I was pointing out that the lower end of the failure, the force a screw can hold is greater for the longer screws and its at a level that could cause belay failure (especially for a factor 2 fall) at a around 8 - 10 kn.
Possibly, but that's not what their data shows. A single screw out of 113 17cm screws failed at a lower threshold than a single screw out of 86 22cm screws, is what their data shows. They also show that a 13cm screw failed at 10.6 kN - but I'm not about to start replacing all my 16cm screws with 13cm ones - I'm not going to draw conclusions from a single sample.
In the absence of more useful data - say the 10th percentile values - I maintain that the difference in average performance between the 17cm and 22cm screws is negligible enough to not be a cause of concern.
As for screws in cruddy ice - well then all bets are off. Maybe someone should do an experiment in ice that more accurately reflects Scottish conditions, but for an experiment to be useful we do need to eliminate some of the variables - the obvious one being the condition of the ice.
I did find the observation about horizontal vs vertical placements interesting, though.