I am not a sufficiently expert statistician to be certain, but I think
the UK data leads to two conclusions about tug upset fatal accidents:

1. There have been too few such fatalities - I think about 6-10 over 30
years - to draw conclusions with a high probablility of being certain of
the correlation - and I can't stipulate how "high" is high;

2. Notwithstanding 1. above, as far as I know 100 percent of UK tug
upset fatal accidents in the last 30 years happened with belly hooks. We
changed our procedures and recommendations before we could gather more
data and satisfy statistical pedants with some more fatalities which
might have improved the correlation calculations. Since the changes,
fatal tug upsets have almost entirely disappeared from the UK fatal
accident reports.

There have been tugging accidents other than upsets, with nose hooks as
well as belly hooks, but these do not affect such inferences as one can
draw from 1 and 2 above.

By the way, I fly mostly a Ka6E with a belly hook. I am very careful
not to kill my friends who tug, being all too well aware of the danger.
One of the changes was to alter our preferred tow position, as has been
referred to by others, to only just above the prop wash - termed the
"low High-tow" position, IIRC. Before the changes, we normally kept the
glider at or slightly above the tug height once established on tow.

Chris N.