Hi all
More a question for clients I suppose but as ever all outlooks welcomed.
I'm wondering how any one in long term contracts is going to address the
prickly issue of likely increased costs of MEWPs etc especially as not one
MEWP will cover all eventualities. I'm at the thinking about it stage so,
unhelpfully, I have nothing to contribute.
John
-----Original Message-----
From: chris.hastie [mailto:Chris.Hastie@xxxxxxxxxx.gov.uk]
Sent: 01 December 2004 12:41
To: UK Tree Care
Subject: RE: Hazard Rating Calculation
On 01 December 2004 12:04, Robin Hassall wrote:
I'm not clear what sort of system you are referring to Robin.
However, I would suggest that if there is some chance of a passer-by
passing by, then the target score is not zero. It may be very small,
but it is not zero. That is the difficulty.
The system I am referring to is THREATS as devised by Julian
Forbes-Laird and which proposes the use of a formula which multiplies
the 3 parameters together without any provision to prevent a zero
answer.
I'm am not specifically familiar with the figures used by THREATS, but
perhaps you should consider some alternatives if the methodology doesn't
stand up to detailed scrutiny. It is certainly not a system with
widespread and uncritical acceptance within the industry.
If the probability of someone / thing being there when the tree fails
really is nil, then it is quite right that the final 'hazard rating'
be nil.
Where do you find such places in reality? Airports? Railway lines?
but even then people are around.
This is not the point. If such a place really existed, then it would be
valid to rank the target as zero. In reality, whilst a suitable figure
might move towards zero, it rarely reaches it other than on Mars.
I agree that it is not wrong to allow the inspector to rate a risk as
zero so long as that zero does not zero everything else by its use
in a formula.
No. You are missing the point. If the probability of a target being
present when the tree fails is zero, then the risk posed by the tree is
zero. This is not arguable. It is the nature of probability.
If the probability of there being target is non-zero, then it should not
be entered as zero. If THREATS suggests otherwise it is flawed.
By setting a base value of one for each parameter as shown
below the problem
is easily circumvented and the inspector can rate risks as
zero according to
the THREATS system without fear of 'deleting' all the other values
scored when it is applied in the formula.
Setting a base value of 1 skews everything. a.b.c != (a+1).(b+1).(c+1)
no matter what the values of a, b and c.
An unfortunate passer by is killed by the falling branch.
I.e. the target should not have been rated zero.
Yes! That is why I am proposing the formula should be changed.
NO. Do not amend the formula. Amend the data you input. If your users
can not tell the difference between zero and very small, they should not
be undertaking the task.
It
was an unlikely
event but in my opinion to allow that tree to be scored as
having a Hazard
rating of zero would be quite wrong and misleading.
What's the difference? Do it right and give the target a rating of
.000000000001. Then the hazard rating is non-zero, but it is still so
small that you will take no action. If the target occupation is that
low, then taking no action is legally defensible.
There is a huge difference!!
If you were to put in such a figure it only emphasises my
point as when
applied in the present formula it would suggest the overall hazard to
be infinitesimal when clearly it is not!
Yes it is infinitesimal. You're really not getting this are you? If the
risk of there being a target is tiny then that WILL push down the risk
of damage, perhaps as far as infinitesimal. If only one person walks
past the tree once every ten years, and it takes 5 seconds to get
passed, what is the chance of a person being around when the tree fails?
1 in 63115200, that's what. Or about 0.000000015. Considerably less than
the chance of a lottery ticket hitting the jackpot. It doesn't matter if
it's a dead cert that 5 tonnes of tree is going to come crashing down
soon. The chance of it hitting someone IS infinitesimal. It is entirely
reasonable to leave the tree. In fact, it would be entirely reasonable
with a tree like that not to bother inspecting it all.
Yes, it might kill someone. And I might be a millionaire by this time
next week. But I'm not holding my breath.
And to the extent that the probability of damage is infinitesimal and
negligible, it is perhaps a valid approximation to call it zero.
I know it sounds far fetched and that the user should employ
more common
sense but, because I work with computer software, I know how
easy it is for
people to key in uncertain data and expect miracles from a
report!!
Which is why the user should be a competent arb who knows what they
are doing and fully understands the implications of that.
Then why should the paper I have in front of me advocate this
procedure if it contradicts training given to a competent arb?
I couldn't possibly comment.
--
Chris Hastie
Strategy Officer (Arboriculture)
Warwick District Council
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