In 25 trees risk methods I reviewed, I have only found one that does not
directly require an estimate of Pf. The main difference between QTRA and
other risk methods' allocation of a probability of failure is that most
qualitative methods e.g. M&C use somewhat vague qualitative terms with
an implied level of uncertainty (e.g., low, medium, etc.), whilst QTRA
or other quantitative methods tend to use point estimates hence imply a
level of precision that cannot be supported by any evidence (the Pf is
still an assessor's subjective opinion). Quantitative risk assessments
in other fields attempt to address uncertainty by using ranges estimates
not point estimates.
Pf tends to be the weakest area for arborists, and regardless of the
assessment method different assessors assessing the same tree come up
with wildly varying estimates, even with simple 4 input fields as found
in M&C it is not uncommon for assessors to vary across three fields. The
exception tends to be very obvious defects (e.g., large hangers).
-----Original Message-----
From: Chris Hastie [mailto:Chris.Hastie@xxxxxxxxxx.gov.uk]
Sent: Friday, 18 April 2008 10:07 PM
To: UK Tree Care
Subject: RE: Difference between QTRA and THREATS
Reply interwoven with original for context.
On 18 April 2008 08:52, Jerry Ross wrote:
Up to a point, Lord Copper.
The third factor within QTRA (probability of failure) is essentially
predictive. The user who claims that 'there's a 1 in 2,500 risk of
this tree failing within the next 12 months'
is, in essence, making a prediction
That would not be my understanding of the word 'prediction'. Prediction
implies they can work out which of the 2500 identical trees is going to
fail.
What's more, in saying that a tree has a 1:2500 chance of failing you
are not actually saying that if you had 2500 identical trees, one would
fail. It is quite possible that none will fail, or two, or all of them.
And possible even to work out the probability of those different
outcomes. Just as if you throw a die 6 times you are not guaranteed to
get a 6, even though the chance of getting a 6 on each throw is 1:6.
I won't disagree with you that picking the probability of failure in
QTRA is one of its weakest areas, because we just don't have those data.
Hence when anyone says "let's collect data on failed trees" I say "let's
collect data on trees that don't fail too". We need both to improve
this.
It's also worth noting that THREATS requires the practitioner to make
some sort of estimate of the probability of failure too. As does Matheny
and Clark's method, the USDA method and I dare say others. In fact, it
is difficult to conceive of a risk assessment method that doesn't
require some estimation of the probability of an event. The difference
is that QTRA expresses that as a probability, whilst many others
effectively mask the fact that it is probability of failure that you are
estimating.
--
Chris Hastie
Strategy Officer (Arboriculture)
Warwick District Council
Looking for a tree surgeon? Visit
http://www.warwickdc.gov.uk/cya
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