Tree Managers : Public Discussion

PDB

Because I do not wish to present a wholly biased view, I posted your message on the User Discussion List where one of our Australian users responded and agreed that I could transfr his response here.
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Sorry,

What a load of crock! QTRA is not about predicting the future and all models rely on past data since it is impossible to this stage to look at future data.

To suggest that climate change may potentially cause significant changes would certainly be true if the change were rapid but the evidence is that the change in the next 12 months will be gradual and based on figures for the last decade the temperature will continue to fall by 0.05 degrees each year (Global cooling?). Not sure how that will affect anything!

The assertion "just because we don't expect something to happen doesn't mean there isn't a 50% chance that it will" is ludicrous! QTRA is not about expectations it is about probability and if the chance of it happening is 50% then the odds are 1 in 2. My city has not been subject to a 1 in 100 flood event for over 120 years. What is the odds of getting a 1 in 100 flood in the next year? That's right 1 in 100 yet our expectations are that we haven't had one for ages so one is due soon! And if we get one next year the odds of having one the year after is 1 in 100. We certainly wouldn't expect to get two floods in a row but expectations do not alter probability. Probability, if we were entirely objective, should alter our expectation.

As far as freak storms are concerned I think there are really good models available and Sydney and other capital cities around the world have been and will be hit by freak storms. Correct me if I am wrong but we are not trying to consider "all acts of God".

When a 1 in 100 wind storm hits trees are going to be damaged. When a 1 in 1000 wind storm hits lots of trees are going to be damaged but to be honest trees are still one of our lowest risks at these times. For those who may not have considered it a tornado represents pretty esteemed winds, they kill people and whilst trees debris may be some of the debris that gets thrown around so do cars, roofs and even whole buildings. I still take my odds with trees and a reasonable defence that acts of God will still deemed as acts of God.

Mark
----- Original Message -----
From: Mike Ellison
To: qtra-users@lists.qtra.co.uk
Sent: Thursday, November 06, 2008 8:30 PM
Subject: [qtra-users] Message on QTRA public discussion forum


The following message has been posted on the QTRA Public Discussion Forum. I'm posting it here to make QTRA Users aware of the Public Forum and also I wonder whether anyone might be interested in replying to postings. The forum has been heavily bombarded with spam and for this reason every post needs to be moderated by me, which in turn sometimes slows down the time that it takes for a message to appear on the webpage.

Mike Ellison


Re: Accuacy of data
Posted by: PDB (89.145.205.172)
Date: Thursday, 06-Nov-2008, 08:30:20


Through my own experience of dealing with hazard trees over twenty years, it is my carefully considered opinion that QTRA is misleading in its use of the term 'Quanitified'; quantified risk assessment in other fields is based on complex statistical modelling. In comparison, the data we have on probability of failure is very poor. There are powerful factors that remain outside of predictability e.g. the weather and risk of hidden factors would also need quantifying.

In this respect, QTRA is a 'qualitative' assessment, based on broad objective criteria that can be applied to ascertain priority based on reasonable practicability.

The risk is that applying a statistical approach can lure the duty holder into a sense of security. If they percieve the risk to be low, routine tree managment can be neglected. This could offset risks to a later stage at a higher level with reduced options - neglected trees are difficult to safely retain and manage. The scurrent statistics of death may be nothing more than the result of twenty years routine tree maintenance to BS3998:1989.

The world financial crisis is a rude remider of how quantified risk assessment in the banking system failed to assess the net effect of various risks compounding one another to create serius consequences, and failure to act cautiously with such risks because they were perceived to be too remote.

There are parallels with tree risk assessment; just because we don't expect something to happen doesn't mean there isn't a 50% chance that it will. That point needs to be stressed to the duty holder for them to decide if they are prepared for the possible consequences. I feel the name and use of odds of QTRA system doesn't explain the brevity of this issue.

Use of statistics to draw odds, is a little like driving down the motorway looking in the rear view mirror. Climate change and the weather are big variable factors on tree risk assessment (always assuming a target), and that requires caution.

Regards
PDB


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