Acer ventura

The Likelihood of Failure and Impacting the Target Matrix with value ranges. I've removed the ? for the lower value of Unlikely because it being the lowest of the likely descriptors it has to stretch off to infinity and beyond. Does this make sense? Any comments? I'll move on to the Risk Matrix on Monday and take on board any revisions based on what comments I get back on or off forum. Cheers Acer ventura

Unlikely Probable & Low 1/1 - >1/2 x <1/2 - ? = <1/2 - ? Possible & Medium 1/2 - ? x <1/1 - >1/2 = <1/2 - ? Possible & Low 1/2 - ? x <1/2 - ? = <1/4 - ? Upper values for Improbable and Very Low not clear, so cannot work out the values for that row and column. Likelihood range for Unlikely from the three cells that can be worked out. Highest value in Unlikely range = Less than <1/2 Lowest value in Unlikely range = ? Cheers Acer ventura

Somewhat likely Imminent & Low 1/1 x <1/2 - ? = <1/2 - ? Probable & Medium 1/1 - >1/2 x <1/1 - >1/2 = <1/1 - >1/4 Possible & High 1/2 - ? x 1/1 = 1/2 - ? Probable & Medium Somewhat likely cell, less than <1/1, greater than or equal to >1/4, is the most explicit and clearly defined cell in the Risk BMP. Lowest value in Somewhat likely range will be lower than >1/4 because the lowest values in Possible and Low are likely to be less than 1/2, so I’ve marked it ‘?’ Likelihood range for Somewhat likely from the three cells Highest value in Somewhat likely range = Less than <1/1 Lowest value in Somewhat likely range = Greater than or equal to >1/4? Cheers Acer ventura

Likely Imminent & Medium = Likely 1/1 x <1/1 - >1/2 = <1/1 - >1/2 Probable & High = Likely 1/1 - >1/2 x 1/1 = 1/1 - >1/2 Likelihood range for Likely from the two cells Highest value in Likely range = 1/1 Lowest value in Likely range = Greater than or equal to >1/2 Cheers Acer ventura

Very Likely Imminent & High = Very Likely 1/1 x 1/1 = 1/1 Cheers Acer ventura

I’ve been getting some off-forum emails about this thread. It didn’t occur to me to mention it when I started it, but if you want to join in, have an opinion, or some insight to share, but are too shy or reluctant to post on here then please email or pm me. Warning. This next section can all too easily mess with your head. Not just the values but the words as well. I know it did mine when putting it together. It’s a buttock-clenchingly tedious but necessary exercise. With the range values that we’ve got so far for Likelihood of Impacting the Target and Likelihood of Failure, I’m going to populate the range values in the cells of the Likelihood Matrix for Very likely, Likely, Somewhat likely, and Unlikely. <<Missing matrix>> I’m going do a post each for Very likely, Likely, Somewhat likely, and Unlikely, so they can be referred to separately, and then one for the completed Likelihood Matrix. So, in the unlikely event that you don't find the workings out fascinating, you can skip to the last one if you want, and check back to the other posts if you want to know how the range values were derived. Cheers Acer ventura

Hi Jules Serious answer... Didn't I explain 'why' in the opening post? If not, expand more on what you mean by 'why?', and I'll try to answer. Cheers Acer ventura

Hi Craig Good to get another take in it from a TRAQ arborist. I don’t think you make light, but highlight an important issue. That being, the lack of clarity that can be inherent with subjective and ambiguous descriptors for input and output ranges with risk matrices. Inherent subjectivity and ambiguity in risk matrices is something the Risk BMP highlights in its Tree Risk Assessment Basics section, where it goes on to stress the importance of clear explanations and significance of the definitions with the ratings for likelihood, consequences, and risk. Another point related to this is there’s some research that even when subjective descriptors are given with clear parameters to reduce the ambiguity, the user’s personal cognitive bias of the word's definition can override how the range is meant to be inputted. To illustrate, when a ‘Probable’ event is defined as being greater than >90%, the user’s own interpretation of what ‘Probable’ means to them can be what they actually apply. So, someone who is risk averse could regard ‘Probable’ as being greater than >50% and apply this definition despite the guidance that 'Probable' means greater than >90%. I understand why you’ve gone with your own words so you can make sense of what you’re making a decision on with Likelihood of Failure, but perhaps you should be careful here. For example, I struggle to see how your ‘Might’ is the same as the Risk BMP’s ‘Probable’. And isn’t ‘Might’ the same as ‘Could’? Cheers Acer ventura

Likelihood of Failure Likelihood of failure over a year to allow comparison with annual risk thresholds described in ToR, or other measured annual risks. Imminent A no brainer at 1/1 and within a much shorter time frame than one year. Probable “failure may be expected within the review period.” Probable range has an upper value of 1/1. Potential conflict of Imminent and Probable ranges not being explicit and overlapping, but Imminent is over a much shorter time frame than one year. Lower value of Probable has to be greater than >1/2. Possible “Failure could occur, but is unlikely” Highest value of Possible range must be 1/2 because the lowest value of Probable is greater than >1/2, so the ranges are explicit and don’t overlap. Lowest value of Possible not clear. Improbable “..not likely to fail” Highest value of Improbable not clear Imminent = 1/1 Probable = 1/1 – Greater than >1/2 Possible = 1/2 - ? Improbable = <? I can’t find a boundary between Possible and Improbable to define the lowest value of Possible and highest value of Improbable. To me, Possible’s lower value of “unlikely” means the same as Improbable’s “not likely” and I can’t separate the two. Any suggestions? Or comments about the ranges for Likelihood of Failure? I'll put together the outcomes of the Likelihood Matrix tomorrow, and take on any suggestions for Likelihood of Impacting the Target and Likelihood of Failure ranges in the meantime. Cheers Acer ventura

Likelihood of Impacting the Target Quoted text is from the BMP and/or training manual. I’m going to use the symbols < > for less than and greater than respectively, but will include the wording as well because the symbol’s meaning can be difficult to immediately interpret for some. High “…most likely impact the target” “Constant occupancy…24hours a day…7 days a week” A point value. Not a range High is 1/1. Medium “as likely to impact the target as not” Lowest value of the Medium range is 1/2. Parked car occupancy at 14 hours is described as “frequent – large proportion of a day - ‘Medium’.” Medium can be greater than or equal to >1/2, and must be less than High <1/1 so the ranges are explicit and don’t overlap. Low “not likely…will impact the target.” Highest value of Low scale range has to be less than Medium <1/2; just below the lowest value of Medium so the ranges are explicit and don’t overlap. Lowest value of Low range is not clear. Very Low “likelihood…of impacting…is remote” Highest value of Very Low is not clear. High = 1/1 Medium = Less than <1/1 and greater than or equal >1/2. Low = Less than <1/2 - Very Low = Less than <? This is far as I’ve got with Likelihood of Impact. Anyone with suggestions for where ‘Low’ ends and where ‘Very likely’ begins; either as hours/minutes/seconds in a day, a percentage, or a probability? I’ll post the Likelihood of Failure rankings tomorrow. Cheers Acer ventura

Hi In February of this year, 8 year old Bridget Wright tragically died when she was hit by a branch that failed from a tree at her school in Pitt Town, north-west of Sydney, Australia. On the back of that incident the Department of Education in New South Wales produced a directive to inspect trees in all state public schools for safety. In order to meet the criteria of the directive arborists had to be qualified to Level 5, and be a QTRA Registered User or have completed the ISA’s Tree Risk Assessment Qualification (TRAQ); TRAQ is founded on the ISA’s Best Management Practices - Tree Risk Assessment (‘Risk BMP’). I was in Sydney, and other cities in Australia, in March delivering QTRA & VTA workshops, and was asked by a number of TRAQ arborists whether there was scope for QTRA and the Risk BMP to work in tandem. This point has also been raised by some QTRA Registered Users, who are ISA members, and were looking to become ISA TRAQ accredited. During the Australian trip, and by email and phone after it, I’ve been discussing the Risk BMP with arborists who have attended both courses, and going through the Risk BMP publication along with the manual that accompanies the TRAQ training. What I’ve been looking to try and do is see where the common ground is, and what the Risk BMP risk rankings of ‘Extreme, High, Moderate, and Low’ mean in terms of actual risk. By way of background. QTRA quantifies tree risk using Monte Carlo simulations and a manual or software calculator to produce a Risk of Harm as a probability, which is the ‘risk assessment’. The level of risk can then be compared to levels of tolerable and acceptable risk in the Tolerability of Risk (ToR) Framework by the risk owner (client/manager) who makes the ‘risk management’ decision. The QTRA approach to tree risk assessment, including an advisory on risk management, is outlined in the QTRA Practice Note; Quantified Tree Risk Assessment The Risk BMP uses two matrices. A Likelihood of Failure & Likelihood of Impacting the Target Matrix, which produces a ‘Likelihood’ ranking. Then the Likelihood ranking is fed into a Risk Matrix, where ‘Consequences’ are also ranked, to produce a qualitative risk ranking of ‘Extreme, High, Moderate, or Low’, which is the ‘risk assessment’. The client then manages their level of tree risk at Extreme, High, Moderate, or Low depending on their ‘risk tolerance’, and this constitutes the ‘risk management’ decision. Those of you who are unfamiliar with the Risk BMP can read about it in Arborist News here; Risk BMP I’ve reproduced the two Risk BMP matrices below, which I've abbreviated to the 'Likelihood Matrix' and 'Risk Matrix', and coloured the cells to make it easier to identify the cell categories in each matrix. What I would like to do, with the help of TRAQ arborists and anyone else who would like to chip in who is familiar with the Risk BMP, is to go through the process of working out where the risks of ‘Extreme, High, Moderate, and Low’ sit in ToR. I’m going to break this up into easy to digest bite-sized pieces and will start the next post by looking at ‘Likelihood of Impacting the Target’ in the first matrix. Cheers Acer ventura Update - There is now a formatted and referenceable summary of this thread available as a pdf document, which can be downloaded at the first post of the new thread here: http://arbtalk.co.uk/forum/general-chat/88697-tree-risk-assessment-traq-qtra-compatibility-common-ground.html .

Hi Awhile back I had a few off forum emails about whether I would discuss the evolvement of QTRA version 5 after it was released last year, in a similar manner to the QTRA Q&A thread I started here; http://arbtalk.co.uk/forum/general-chat/53668-quantified-tree-risk-assessment-qtra-questions-answers.html Fresh back from a QTRA & VTA tour downunder, and now mentally and physically back in the right time zone, I’m going to start one here. Version 5 of QTRA is it's most significant evolution since it was put it together in the 1990s, and the main improvements are; Monte Carlo Simulations The Risk of Harm is calculated using Monte Carlos simulations, which are built into the engine of the system. In short, Monte Carlo simulations mean QTRA calculators work out the ‘most likely’ Risk of Harm from 10 000 possible outcomes for each combination of Target, Size, and Probability of Failure range. The benefits of calculating the most likely outcome are threefold. Risk aversion (worst case scenario) is avoided and valuable resources are therefore not wasted on unnecessary tree work. Being risk prone (best case scenario) is avoided, and its associated vulnerability to claims of a breach in the duty of care. Perhaps most importantly, the inherent uncertainty that is ever present when assessing the risk from tree failure is embraced and accounted for. Risk of Harm Risk of Harm outputs are colour-coded around an easy to understand traffic light theme, red, amber, yellow, and green. The colours relate to the long-standing and internationally recognised tolerable and acceptable thresholds for risk in the Tolerability of Risk (ToR) Framework. With version 5, a ‘yellow risk’ can be considered tolerable and imposed on the public if it is As Low As Reasonably Practicable (ALARP). A ‘green risk’ is acceptable and it is not necessary to demonstrate any benefits to the risk. Risk Manager Guidance This has been beefed up in the Practice Note to enable risk managers to consider not only the level of risk from their trees, but the costs of reducing the risk and the benefits lost from felling trees. By adopting advisory risk thresholds within the Practice Note, risk managers are able to pursue simple and robust tree risk management principles. Probability of Failure Calibration Exercises Selecting a Probability of Failure range is the most uncertain and assessor dependent part of a tree risk assessment. For a number of years now, during the field exercises of both the QTRA and VTA days, we have spent time calibrating Probability of Failure ranges with the delegates. Along with improved ‘benchmark’ guidance in the user manual, more time is now spent in the field with these exercises and greater consistency is being achieved. The VTA day, in particular, focuses on the underpinning principles and practical application of selecting a Probability of Failure range for a tree or branch with confidence. New Calculators There are new manual and software (windows and android) calculators. Both have been re-designed to make them easier to use. Easier to Use With Monte Carlo simulations driving the Risk of Harm outputs there’s no scope to mess about trying to refine the inputs and potentially ending up in a maths cul-de-sac. The assessor merely needs to choose the correct Target, Size, and Probability of Failure range on the new improved calculator, and the colour-coded Risk of Harm is generated for them. The colour of risk then informs the guidance to the risk manager. The Practice Note is available here. Quantified Tree Risk Assessment If anyone wants to discuss any the above, other elements of QTRA v5, or indeed any other tree risk issues, then please feel free to fire away with any questions you may have, and I’ll do my best to help out. Cheers Acer ventura

Hi Tony My reply to this one got lost somewhere between Singapore and Perth, and I think this may address the point you raise here. A 1/10 000 RoH represents anything between and including; a 1/10 000 likelihood of losing a life a 1/1 likelihood of injury equivalent to a 10 000th of a life. Cheers Acer ventura

Hi Jules If you’re using QTRA, the ‘actual RoH’ here is 1/1 000 000. 1/975 000 is not a RoH. It’s an incomplete calculation of the RoH, for the reasons I've explained previously. If you didn’t want to use QTRA to assess tree risk, and put together another method to measure the risk to such a precise probability of 3 significant figures and got 1/975 000. Then, with a risk at 1/975 000, and taking the Value Of Statistical Life at £1 000 000, action would only be required if it cost less than £1.03 (rounded to pennies) to bring the risk down to 1/1 000 000. If you have to spend more than £1.03 the risk is ALARP and no action is required. If you go ahead and spend more than £1.03 you’re being ‘grossly disproportionate’. Indeed. With QTRA a RoH of 1/900 000 is ALARP unless the cost to reduce the risk to 1/1 000 000 is less than £1.11. Cheers Acer ventura

Common law duty of care… “…to take reasonable care to avoid acts or omissions which you can reasonably foresee would be likely to injure your neighbour” …is Lord Atkin from Donoghue v Stevenson, which according to Charles Mynor’s ‘The Law of Trees, Forests and Hedgerows, “…is still the leading case on the area of civil liability.” The ‘duty’ in the Scottish 1960 Act doesn’t look ‘different’ to the 1957 or 1984 Act to my untrained legal eyes. We have that word ‘reasonable’ again in relation to the level of ‘care’ expected from the duty holder. It seems to me that a duty holder in Scotland' date=' who manages the risk from their trees to 1/10 000 because of all the benefits they convey, so long as the risk is ALARP, would be exercising [b']reasonable care[/b]. Cheers Acer ventura

Hi Jules No matter how accurate the inputs are, and as demonstrated earlier in the thread we can get very accurate traffic figures, the output RoH is only ever expressed to 1 significant figure. It’s not physically possible to get more than a 1 significant figure RoH using the manual calculator or the software calculator, no matter what level significant figure values actually go into the calculation. If you’re approaching a RoH threshold, with input values that are not close to the highest value of the broad ranges, and want to refine the risk by multiplying the 3 components with a conventional calculator then you round the end RoH to 1 significant figure yourself. If you’ve cobbled together an excel spreadsheet, then you set it to round to 1 significant figure. The rounding to 1 significant is never ‘ignored’. You may recollect from earlier, we always round the RoH to 1 significant figure because no matter the accuracy or levels of significant figures of the Target or Impact Potential (Size of Part), the PoF component is expressed within a range and to 1 significant figure. Cheers Acer ventura

Hi Jules I appreciate that but you raised it and the exact wording is really, really important. There's a huge difference between the meaning of 'foreseeable' and 'reasonably forsee would be likely'. It's one of the foundations on which QTRA is built, which is what we're talking about on this thread. Cheers Acer ventura

Hi Jules For Harm, you mean Impact Potential (Size of Part). For reasons of being ‘proportional’ and ‘reasonably practicable’ within the risk assessment, with the manual calculator and the broad ranges in the software calculator, it is the highest values within the broad ranges which are multiplied. If the actual input figures are some way from those highest values in the range, then the RoH will be an overestimate of the risk. It will err on the side of safety. In most cases this does not matter because the risk is so low. As such, if refining the inputs would result in the RoH being even lower why waste time and effort coming out of the manual calculator to refine the risk? Where the resulting RoH exceeds or approaches a risk threshold – say 1/10 000 – which might otherwise prompt some tree work to control the risk, then the risk assessor is advised to look closely at the inputs because of the very reason that we’re taking the highest value from the broad ranges. It might be that refining the RoH by, for example, inputting the actual Target Value, if you have confidence in its origin, results in a RoH substantially lower than 1/10 000. This can occur with high Target Values, and there’s an example of Target Range 1 (1/1 - 1/20) refinement to illustrate this point in the QTRA training day. Have a look at ‘Accuracy of Outputs’ section on p. 7 of the QTRA Practice Note Quantified Tree Risk Assessment I think this point has been already been addressed earlier here. http://arbtalk.co.uk/forum/general-chat/53668-quantified-tree-risk-assessment-questions-answers-4.html#post825414 ...the Goode post at Post 1, and throughout the thread. As an aside, on this issue of highest values within a range, one of the many things I found interesting when analysing other tree risk assessment systems, is that they all use bands or ranges, but don’t disclose whether it’s the highest value of that range, an average (and if so what average), or the lowest value, that is determining what the eventual risk outcome is. One of the many reasons that QTRA chimed with me when I first encountered it was that all the components, how they are put together, and why they are put together in the manner in which they are is entirely transparent. Cheers Acer ventura

I don’t think it does. Common law duty of care “…to take reasonable care to avoid acts or omissions which you can reasonably foresee would be likely to injure your neighbour” Occupiers’ Liability Act (1957 & 1984) “…a duty to take such care as in all the circumstances of the case is reasonable to see that the visitor will be reasonably safe in using the premises for the purposes for which he is invited or permitted by the occupier to be there.” The words ‘avoid’ and ‘foresee’ are importantly qualified by a range of ‘reasonables’, and an all-important ‘likely’, and they aren’t in the Occupiers’ Liability Act at all; though notably there’s another couple of ‘reasonables’ there. In relation to the UKTC debate about this: Foresee = Prediction Reasonably foresee would be likely = Probability Beyond that, and in particular relating to the Health & Safety at Work Act (1974), and various risk assessment and risk management guidance, we also need to embrace ‘reasonable practicability’ and ‘proportionality’. Cheers Acer ventura

As well as the HSE, The National Tree Safety Group (NTSG), the British Medical Association, The Royal Society, NASA, and all other space agencies (though Iran and North Korea are probably doing their own thing) all propose the 1/10 000 or 1/1 000 000 levels of risk as being ‘tolerable’ or ‘acceptable’. In QTRA language, the ‘H’ in RoH is ‘Harm’ not D ‘Death’ To quote QTRA Guidance; “QTRA Risk of Harm 1/10 000 An advisory threshold of ‘tolerable’ risk where it is imposed on the public in the wider interest; providing the risk is ALARP. QTRA Risk of Harm 1/1 000 000 An advisory threshold of ‘acceptable’ risk. No need to consider whether the risk is ALARP." 1/10 000 is not ‘ingrained’ in QTRA calculations. QTRA measures the components of tree risk as probabilities in order to express the ‘Risk of Harm’ (RoH) as a probability so that the risk manager/owner can consider it against these published thresholds. They don’t have to manage their tree risk to 1/10 000 or 1/1 000 000, they could choose any level of risk they wish. All we are suggesting is a reasoned and reasonable basis to adopt these thresholds for the tree manager/owner, and that the risk assessor propose them in their reports.

Hi I'm about to post a bundle of replies to the open questions. I’ve tried to break the issues down into easy to digest posts, so that they’re easier to follow and reply to. Hi Jules I don’t wish to sound impolite, but earlier in this thread you were very adamant that you didn’t want to discuss your take on quantifying tree risk. I have some serious reservations about some of the things you are saying, or claiming to be able to do, in your last few posts, as you gradually reveal some of the stitches and patterns of your home-knitted approach to quantifying tree risk. I’ve highlighted a couple of points of concern above. Can we agree that you don’t say what your take on it is if you don’t want me to comment on it? Unless you’ve changed your mind and do want to discuss your version? Cheers Acer ventura

Hi Jules I am back now but haven't had the time to respond. Hopefully, I'll post some replies this week. Cheers Acer ventura

Hi Jules Apologies for the delay in getting back to you on this, and thanks for your patience. I’ve finally found a bit of time whilst at my stopover in Singapore to reply. However, by way of warning, I’m likely to be even slower to respond over the next week and a half than I have recently because upon arrival in Australia I’m being whisked off by friends for acclimation and elocution lessons in ‘Strine’ over the weekend in an internet free outback and beyond place I believe is called Wolf Creek. Then next week, in Perth, we’ve had to put on an additional QTRA workshop to meet demand. So, along with the VTA and QTRA Update workshops, this means by a week Friday evening, having run 4 one day workshops back to back, the risk that I’ll be completely and utterly mentally ‘fecked’ is between 1/1 – 1/10. I'm still happy to answer questions anyone wants to ask, but the speed of correspondence might be more akin to sending telegrams in the age of steam, over the next few weeks.

Hi Simon Thanks for that. If the legal cases I cited interest you, there’s a wealth of information at the QTRA Case Law page QTRA Case Law I did draft most of a reply to your bending moment question, but I’m trying to see whether I can edit and speed up a video of a tree ‘mass damping’ to illustrate the point. Cheers Acer ventura

A very helpful lady called Ellie, from Lancashire County Council’s highways, has kindly sent me the usual incredibly accurate traffic data, which these departments regularly capture, from a counting point at Ruff Lane to the west of Vicarage Lane, which confirms the T value is in Range 2. From a virtual stroll down the road, it looks like the trees within falling distance of the Ruff Lane are in IP Range 2. This means for any of them to be regarded as an unacceptable level of risk, the PoF has to be as high as the 1/100 - 1/900 Range 3. For whole tree failure, that means it must be 10,000 times more likely to fail than our notional 1/1,000,000 optimised tree. This is a tree that would be so defective, its symptoms are likely to be readily apparent to a passing arborist from the comfort of their own car, and may even cause a momentary loss of concentration and drift whilst swearing at how ‘fecked’ it is. I’ve aligned the manual calculator to show the 1/4,000 RoH from a 1/100 - 1/900 PoF Range 3. Even if some of the trees within falling distance of Ruff Lane exceed IP Range 2, and crossed over to IP Range 1, the PoF would still have to be as high as the 1/100 - 1/900 Range 3, because even if it was as high as 1/1,000 - 1/9,000 Range 4, the RoH is 1/20,000. One of the Rangers, Dan, told me the main paths of the site are walked every two weeks by a Ranger, and they are all trained in basic tree hazard recognition. Apparently, the most common form of tree failure is snow-laden upper lateral branches of the Scots Pines. However, the signage at the entrance points doesn’t have a simple 'ring this number if you think there is a dangerous tree' annotation to it. This is an easy to do thing that would be worth considering because it’s in the HSE’s Sector Information Minute ‘Management of the Risk From Falling Trees’. Having this in place would leave any HSE Inspector with at least one less stick with which to potentially beat the council with in the event of a risk being realised. All this detail can be got hold of and sketched out before visting the site. I’ll explain how we would go about the 'Walkover Survey' when visiting the site later. Cheers Acer ventura