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Found 1 result

  1. Hi Arbtalk RobArb’s thread http://arbtalk.co.uk/forum/general-chat/50770-qtra-im-sorry-i-dont-agree.html has been brought to my attention. It raised a number of questions and had some misconceptions about QTRA, so I thought I would trek over from the UKTC forum, dip my toes into Arbtalk, and chip in to answer some of the questions raised and clarify a few things. By way of introduction, I’m David Evans and outside of my consultancy work at The Arbor Centre I run QTRA and VTA workshops. I’m starting a new thread because the old one died, and it went off topic a fair bit. Please feel free to ask any questions and I’ll do my best to help out. I’ll begin at the beginning, with some of the points raised in RobArb’s first post. I appreciate the thread developed since then, but some of the issues raised are foundations on which everything else is built, so it’s pretty important to get them clear. <<I just can't be drawn into a system that relies on statistics as its main factor.>> QTRA doesn’t rely on statistics as its main factor, it’s founded on probability, which is the language of risk. Risk = Likelihood x Consequences. With QTRA, the likelihood and the consequences of someone or something being hit by a tree, or part of a tree. <<it has been deemed by QTRA that an acceptible risk is 1/10000. This from what i can tell has been derived from historical tree failures>> The 1/10,000 threshold does not come from QTRA, nor does it relate to historical tree failures. In summary, 1/10,000 is a ‘tolerable’ level of risk that can be imposed on the public for the wider good where the risk is As Low As Reasonably Practicable (ALARP); as outlined by the Health and Safety Executive (HSE). This is a really useful defendable threshold for managing tree risk to because it’s a level of risk that has been determined by an independent watchdog appointed by the government on behalf of society at large. It is not a level of risk that has been made up or is the opinion of an arborist. The reason QTRA was created was to enable the user to measure and calculate risk as a numerical probability, so the tree owner/manager can manage their liability and discharge their duty of care to a published and accepted level of tolerable risk. The 1/10,000 that is often cited. Rather than suffer me prattling on about its origins and context, what risk is, the 1/10,000 level of tolerable risk and its position in the HSE’s Tolerability of Risk (ToR) Framework, it can be seen in the first 3 pages of the QTRA Practice Note, which you can be downloaded here; Quantified Tree Risk Assessment The 2007 Goode Judgment from South Australia is interesting for a number of reasons. Not least because the Commissioner criticizes the 'implied precision' of the QTRA risk output and then bizarrely uses QTRA to back up his opinion but makes a fundamental error in applying it. The ‘implied precision’ issue he raised had been dealt with before the Judgment, and the Risk of Harm (RoH) output should not have been expressed with such precision to 4 significant figures, no matter how precise any of the inputs. The RoH with QTRA is expressed to one significant figure only. Mike Ellison did a presentation that included a detailed reply about the Goode judgment which I've extracted and attached. I’ll get onto the ‘subjectivity’ issue tomorrow. Anything to ask in the meantime, then please fire away. Cheers Acer ventura Goode Judgment Comments - Mike Ellison.pdf

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