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Risk Matrix Literature For those of you who are interested in reading a bit more about risk matrices and getting a better understanding of their intrinsic flaws from people who have forgotten more about the subject than I know, here’s some links. The ground-breaking seminal paper that first looked at matrices in detail is by Tony Cox, ‘What’s Wrong with Risk Matrices’ What's Wrong with Risk Matrices? - Anthony (Tony)Cox - 2008 - Risk Analysis - Wiley Online Library The paper is hard work and very mathematical in parts. Kailash Awati does a very good job of distilling the essence of the Cox’s work on his ‘Eight to Late’ blog here. Cox?s risk matrix theorem and its implications for project risk management | Eight to Late Douglas Hubbard, ‘The Failure of Risk Management’ Chapter 7 - Worse than Useless: The Most Popular Risk Assessment Method and Why it Doesn't Work. Here’s an Amazon.com link, which I’ve gone for because it has more reviews than the UK site. The Failure of Risk Management: Why It's Broken and How to Fix It: Douglas W. Hubbard: 9780470387955: Amazon.com: Books Alexander Pickering & Stephen P Cowley, ‘Risk Matrices: Implied Accuracy and False Assumptions, is a good paper and pretty easy to grasp. OHS SJ Internals 9-16 Cheers Acer ventura

With the Risk BMP Risk Matrix, there is such intense ‘range compression’, overlap, and lack of identity of the ranges in the Likelihood of Failure and Impact axis that it’s not possible to plot the Risk Contours. My best Scientific Wild Ass Guess would be that they are extremely negatively sloped, running almost vertical like the stripes on a zebra because of the probabilities for the risk of a death. This is far as I’ve got in terms of analysis. There's some off-forum discussions going on, and I will attempt a summary latter this week to include elements of those. As always, any views would be very welcome on or off forum. Cheers Acer ventura

Risk Contours We saw these two diagrams on the Betweeness section to illustrate that the risk gradient increases from bottom left to top right. The lowest value for each cell is the bottom left corner. The highest value for each cell is the top right corner. Here’s what the Risk Contours typically look like if the Likelihood and Consequences axis were of a similar scale. The Risk Contours represent a line of equal risk. In the same way that contours on an Ordinance Survey are points of equal elevation. The top right contour is the highest level of risk and left the lowest. The contours are ‘illustrative’ only to demonstrate a problem that all risk matrices suffer from. The geometry of the Risk Contours v cell structure has implications for the actual risk values of the risk rankings. Risk A = High rating Risk B = Low rating Overlay with illustrative Risk Contours. Remove all but one Risk Contour Risk A, the High rating, lies to the left of the Risk Contour Risk B, the Low rating, lies to the right of the Risk Contour However the actual value of the risk is lower to the left of the Risk Contour, and higher to the right of the Risk Contour. Therefore Risk B, the Low rating in the Risk Matrix, is a higher risk than Risk A, with the High rating. The portion of the Low Risk B cell that lies above the contour, is a higher risk, than the portion of the High Risk A cell that lies below the contour. Low risk ratings can be higher levels of risk than High risk ratings. High risk ratings can be lower levels of risk than Low risk ratings. And so forth with Moderate risk ratings. The problem that all risk matrices face is Risk Contours are negatively sloped, running top left to bottom right, whereas the cells are infinitely sloped being constructed of vertical and horizontal lines. Consequently, cells of different qualitative rankings will be bisected by Risk Contours of the same value pitching anything to the left of the contour in a cell at a lower actual risk value than anything to the right of the contour, which will be higher. Cheers Acer ventura

Risk Matrix Betweeness In the Risk Matrix there is one example of Betweeness. Somewhat likely + Minor = Low jumps directly to Likely + Significant = High A Low risk rating jumps to High, skipping over Moderate, with marginal increases in Likelihood that takes Somewhat likely to Likely, and Consequences that takes Minor to Significant. In addition to the Betweeness from the Likelihood Matrix where the upper value of Unlikely (Number 1) and Somewhat likely (Number 2), is the same upper value as Likely (Number 3). The upper value of Low must be the same as the upper value of Moderate, otherwise it could not go directly to High without passing through Moderate. The top value of Low = top value of Moderate Whatever the risk rating is for the ‘Moderate’ range, is also the highest risk rating for the ‘Low’ range, and its respective cells. In the Risk Matrix, the only cells with a ‘Low’ risk ranking in, after adjustment for Betweeness in the Likelihood Matrix, is the Negligible column on the Consequences axis. Here’s the Risk Matrix, expressed in probability ranges, accounting for Betweeness in both the Likelihood Matrix and Risk Matrix. Has everyone been able to follow this? Any points of clarification needed? Please post here, or pm/email me if you'd rather not go public with an opinion. Cheers Aver ventura

Hi Paul Thanks for the feedback. When I was first looking at this I tried to describe 'Betweeness' in an email to a number of people, without diagrams. As I'm sure you can imagine, it was not an easy thing for them to grasp. Did you manage to follow the logic to the end? Cheers Acer ventura

Hi Treeseer Thanks for that insight. The ALARP region lies between 1/1 000 and 1/1 000 000 per annum in ToR. I’m not sure how this quantitative range could be nailed down with a qualitative descriptor like ‘Very low’, unless the numerical probability range is the definition of ‘Very Low’. It would also squeeze ‘Low’ into meaning a risk greater than 1/1 000, and a Moderate risk range would have to be even higher than that. Under such circumstances the ‘middle ground’ would only be ‘middle’ relatively. In reality a Moderate risk being significantly higher than 1/1 000 is surely not ‘middle ground’ in comparison to an annual exposure to common risks. Which also feeds back to part of the problem of claiming to take into account the owner/managers Risk Tolerance. Is the owner/manager’s understanding of what a Moderate risk rating means in terms of Risk Management the same as what a Tree Risk Assessment of Moderate risk means? Given what appears to be the case from this thread, I think the likelihood of that being the case is perhaps ‘Extremely unlikely’? Then there’s also the consideration of how the owner/manager figures out whether the risk is ALARP when faced with an ALARP range in numbers, a fee proposal to reduce the risk in numbers, and a risk rating in subjective and ambiguous words. Many trees, of course, evidently have a risk that is lower than 1/1 000 000, given that the annual risk of death from tree failure in the USA is substantially lower than this. Isn’t it a bit higher than the UK’s 1/10 000 000, at around 1/9 000 000? Which begs the question, what sort of 'Low' would be used to describe risks that are less than 1/1 000 000 that are acceptable without the need to consider whether the risk is ALARP?

What does this mean for the Risk Matrix? If the highest values of ‘Unlikely’ and ‘Somewhat likely’ are the same as ‘Likely’. Then whatever the risk rating is for ‘Likely’, is also the highest risk rating for the ‘Somewhat likely’ and ‘Unlikely’ ranges and their respective cells. I’m going to get on and do some proper work and will post the Betweeness example in the Risk Matrix later. Cheers Acer ventura

What the Likelihood matrix is saying is that the Likelihood of Failure and Impacting is not a progressive increase from; Unlikely < Somewhat likely < Likely < Very Likely Number 1 < Number 2 < Number 3 < Number 4 We have the top of each Likelihood kissing the bottom of the range above it. With the exception of the top value of ‘Very likely’ (Number 4) which kisses no bottoms. However, the top value of ‘Somewhat likely’ (Number 2) is able to jump over ‘Likely’ (Number 3) and kiss the bottom of Very likely (Number 4). The top value of ‘Unlikely’ (Number 1) is able to jump over ‘Somewhat likely’ (Number 2) and kiss the bottom of Likely (Number 3). What this means mathematically is; The top value of; ‘Likely’ (Number 3) = Top value of ‘Somewhat likely’ (Number 2) = Top value of ‘Unlikely’ (Number 1). All of the ‘Likely’ (Number 3) range share the same top values as the ‘Somewhat likely’ (Number 2) and ‘Unlikely’ (Number 1) ranges. The lowest half of the ‘Somewhat likely’ (Number 2) range, share the same values as the mid-range of ‘Unlikely’ (Number 1). Cheers Acer ventura

Likelihood of Failure and Impacting can jump from ‘Somewhat likely’ (Number 2) to ‘Very likely’ (Number 4), without passing through ‘Likely’ (Number 3) where. Probable + Medium = ‘Somewhat likely’ (Number 2) jumps directly to Imminent + High = 'Very likely’ (Number 4) The top of ‘Somewhat likely' (Number 2) is kissing the bottom of ‘Very likely’ (Number 4). To go directly from ‘Somewhat likely’ (Number 2) to ‘Very likely’ (Number 4) without passing through ‘Likely’ (Number 3) means the highest value of ‘Likely’ (Number 3) is the same value as the highest value of ‘Somewhat likely’ (Number 2). Therefore, the top value of ‘Somewhat likely’ (Number 2) = the top value of ‘Likely’ (Number 3). Cheers Acer ventura

Similarly, Likelihood of Failure and Impacting can jump from ‘Unlikely’ (Number 1) to ‘Likely’ (Number 3), without passing through ‘Somewhat likely’ (Number 2) where; Probable + Low = ‘Unlikely’ (Number 1) jumps directly to Imminent + Medium = ‘Likely’ (Number 3) Without bothering to pop in and say hello to 'Somewhat likely' (Number 2) Cheers Acer ventura

Possible + Medium = Unlikely (Number 1) ‘Possible’ Likelihood of Failure - “Failure could occur, but is unlikely” is at its highest value in its range. ‘Medium’ Likelihood of Impacting the Target - “as likely to impact the target as not”- is at its highest value in its range. We're sat in the top right corner of the 'Unlikely' cell The smallest increase in Likelihood of Failure takes the range value from ‘Possible’ – ‘Probable’ (“failure may be expected within the review period.”) The smallest increase in Likelihood of Impacting the Target takes the range value from ‘Medium’ – ‘High’ (“…most likely impact the target”). Probable + High = Likely (Number 3) We're sat in the bottom left corner of the Likely cell Have gone from ‘Unlikely’ (Number 1) – ‘Likely’ (Number 3) Likelihood of Failure and Impacting. ‘Somewhat likely’ (Number 2) has been jumped. The Likelihood of Failure and Impacting value has gone from ‘Unlikely’ (Number 1) to ‘Likely’ (Number 3), bypassing ‘Somewhat likely’ (Number 2). The top of ‘Unlikely’ (Number 1) is kissing the bottom of ‘Likely’ (Number 3). To go directly from ‘Unlikely’ (Number 1) to ‘Likely’ (Number 3) without passing through ‘Somewhat likely’ (Number 2) means the highest value of ‘Unlikely’ (Number 1) is the same value as the highest value of ‘Somewhat likely’ (Number 2). Therefore, the top value of ‘Unlikely’ (Number 1) = the top value of ‘Somewhat likely’ (Number 2).

Betweeness Earlier in thread we bore witness to some apparent flaws in the Risk BMP matrices that are commonplace to many risk matrices. Namely, ‘range compression’ and ‘poor resolution’, with the Likelihood of Failure and Impacting ranges of Unlikely, Somewhat likely, Likely, and Very likely. I’m going to walk you through another common flaw with matrices labelled as ‘Betweeness’ and how it affects the Risk BMP matrices. Firstly, we’re going to look at the Likelihood Matrix and you might want to grab a beverage of choice and clear your head. Likelihood of Failure and Impacting the Target matrix outputs has four ranges increasing from; Unlikely (Number 1) < Somewhat likely (Number 2) < Likely (Number 3) < Very likely (Number 4). I’ve numbered them to help with what’s coming later. I’ve run this by a few people and it can easily mess with your head, so I’m going to try and explain it as clearly and illustratively as I can in a number of posts. I think the numbers help magnify the ‘betweeness’ better than the gaps that allow jumps across the Likelihood words. Risk Gradient - increase in risk rating from bottom left to top right. I’ll look at the risk contours tomorrow. Risk Gradient overlaid onto the Risk Matrix. The lowest value for each cell is the bottom left corner. The highest value for each cell is the top right corner. Cheers Acer ventura

Hi btggaz Risk matrices are really easy to design, which is probably why they are so popular. The problem is most of them, to quote Tony Cox from his seminal 'What's Wrong with Risk Matrices' paper, are 'worse than useless' at measuring risk. When I was doing the research into the ISA Best Management Practices - Tree Risk Assessment & TRAQ thread, one of tests I put myself through was to try and explain what the Likelihood and Risk matrices revealed to someone who knew nothing about risk, trees, or matrices. When I finished, they said, “It’s just colouring squares, isn’t it?” However, as it's an assignment the purpose is to pass, so don't go for more than four inputs on each axis but you should have a minimum of three. Colour the top right cell red, and the bottom left cell green. Describe the worst case scenario Likelihood and Consequences inputs to give you the red top right cell and call it something like Extreme. Or right click Extreme and choose a synonym you like the sound of. Describe the best case scenario Likelihood and Consequences inputs to give you the green bottom left cell and call it something like Low. Or right click Low and choose a synonym you like the sound of. Now all you have to do is fill in the bits between with your choice of words and colours that are in between the top right and bottom left. If you pop into the http://arbtalk.co.uk/forum/general-chat/73829-isa-best-management-practices-tree-risk-assessment-traq.html thread tomorrow, I’m going to start posting what often goes wrong with risk matrices and how it’s impossible to avoid some of the problems, which you might find useful. Cheers Acer ventura

As Low As Reasonably Practicable (ALARP) The QTRA approach to Tree Risk Assessment is to calculate a Risk of Harm as a probability so it can be compared to thresholds of risk in the Tolerability of Risk Framework (ToR). Owners/managers are then able to make Tree Risk Management decisions based on the thresholds and principles in ToR. One of the key elements of ToR is a region where the risk is Tolerable if it is ALARP. There is a substantial conflict in approach between QTRA and the Risk BMP here because the Risk BMP has is no mechanism to determine whether the risk from a tree is ALARP. The consequences of not assessing whether the risk from a tree is ALARP are that the costs to the owner/manager of managing the risk from their trees are not considered. Consequently, the Tree Risk Assessment can not only be unbalanced, biased, and risk averse, but also stray into becoming a Tree Risk Management policy decision on the part of the risk assessor, causing unnecessary and disproportionate expenditure on tree work and a loss of tree-related benefits. ALARP, was born out of the ‘reasonably practicable’ element in the universal common law ‘duty of care’, and international variations of the ‘occupiers' liability’. The purpose of ALARP is to recognise that when the risk from trees falls below a certain level, making trees safer involves costs in reducing the risk to an even lower level. These costs are the financial costs of the tree work, the loss in tree-related benefits, as well as additional risks from the tree work to the workers and public. Such costs should be considered and balanced against the reduction in risk to determine whether a risk is Tolerable. Otherwise, Tree Risk Management can become a disproportionate tail-chasing exercise where there is often a lower level of risk that could be achieved, with no regard to how much it might cost. To not consider ALARP is to be ‘risk averse’ and worship at the unattainable altar of safety, no matter the cost to the client. Assessing whether a risk is Tolerable and ALARP is about being ‘risk aware’, rather than ‘risk averse’, by not only seeing the elephant in the room but by weighing it. I highlighted an example of the problems faced by the tree owner/manager when ALARP is not considered, in the Risk Tolerance section that proceeded this, in the ‘Lower Than Low’ post; In summary. Case study on p.148 of the TRAQ Manual describes a Low risk from a large dead limb over a path in an urban park. The mitigation recommendation is to ‘remove the dead limb to reduce the risk’. It’s explained that if the Risk Tolerance of the client is Low there could be even more mitigation costs for them to bear by restricting access to the path until the dead limb is removed. The reasoning behind the recommendations is explained as “mitigation is easy and inexpensive”. Beyond the issue of why recommend tree work that will cost money to reduce a Low risk to a Low risk. Or that if the Risk Tolerance of the owner/manager is Low even more money is spent restricting access despite the risk being Low, until the tree work is undertaken and the Low risk becomes Low. The work and restriction of access are likely to cost hundreds of dollars to manage what was already a Low risk. Without consideration of ALARP there’s no mechanism to work out whether the financial costs of the restriction in access to the path and the tree work, exceed the benefits of the reduced risk. Moreover, such Tree Risk Assessment decisions have Tree Risk Management implications that any Low risk should be mitigated to a lower risk if the arborist can suggest a way of spending the owners/managers money and the funds are available. Anyone got any ideas as to how to square the principles of ALARP with the Risk BMP? Tomorrow I’ll start on the last section of this journey, which is looking at some of fundamental problems that are intrinsic to risk matrices. This section might offer some explanations as to what appears to be going wrong with the Risk BMP matrices. Those of you who encounter risk matrices day-to-day may find useful analysis in there. Cheers Acer ventura

Hi James I'd encourage you to go for it and see where it takes you. I do like the idea of an open source tree risk assessment method that quantifies tree risk, but I think you'll be surprised how many hours it consumes, no matter how altruistic your time budget. Moreover, the costs of training (which I can tell you from first hand experience is essential) and administering such a thing are not going to be cheap if you want it to have credibility, and not have everyone customise the hell out of it to suit their own biases. I'm afraid it's not of interest to me though because I have some idea of what your journey is going be like, and any contributions I might have been able to make are already in the melting pot of how QTRA can marginally evolve. However, if you produce something superior to QTRA then I'll drop QTRA, use what you've developed, and then look to contribute. As I'm sure you're aware, you do need to be careful of copyright though because I can' t think of a way of quantifying tree risk that isn't going to look suspiciously similar to QTRA. I've subscribed to the thread and will be interested to see how you get on. Cheers Acer ventura

Tree Risk Assessment v Tree Risk Management Policy The owners/managers Tree Risk Management Policy is being determined by the risk assessor. The Risk BMP, in line with ISO 31000:2009 Risk Management – Principle and Guidelines, makes an important point about distinguishing between Risk Assessment and Risk Management. However, the risk ratings of Extreme, High, Moderate, and Low are not only Risk Assessments but are also crossing the boundary into Risk Management Policy decisions, for some of the reasons I’ve outlined in the previous posts. To paraphrase this warning about risk assessors and Risk Assessments straying into Risk Management Policy by David Ball, (Professor of Risk Management Decision Analysis and Risk Management), in his book, ‘Public Safety and Risk Assessment’ (which is a great read for those of you who are interested in the subject).

Is it Safe? Many tree owners/ managers will not have enough knowledge about risk for the risk assessor to determine what their Risk Tolerance is to it. People are notoriously bad at working out what levels of risk they are exposed to. They tend to grossly overestimate the risks of high consequence very low likelihood events such a shark attacks, plane crashes, and terrorist atrocities. And grossly underestimate much higher daily risks such as driving, diet, and household tasks. There’s a wealth of literature from behavioural economists and psychologists to demonstrate that people are simply bad at working out risks. From a discussion between both parties, I think it extremely unlikely a risk assessor is sufficiently well trained, or the owner/manager has sufficient knowledge of how they ‘perceive’ risk, that either can determine what a credible Risk Tolerance is to tree risk, unless they choose Low. Cheers Acer ventura

An Imposition The Risk Tolerance of those who have the risk imposed on them is not considered. Where a tree, or part of a tree, has the capacity to fail outside of the owner/manager’s property, the risk is imposed on others and the risk is not solely ‘owned’ by the tree owner/manager. Tree risk being imposed in this manner is often the case where there are the highest value Targets, and consequently where there are the highest potential risks. The Risk Tolerance of those who are exposed to having the risk imposed on them is not considered. Cheers Acer ventura

Lost in Translation Risk Ratings cannot be compared to any other risks. Extreme, High, Moderate, or Low risk ratings are defined by the inputs and outputs of the Risk BMP matrices, and they only have meaning within the Risk BMP. A tree owner/manager cannot compare the tree risk rating to any other risks they have to manage, or might understand. The risk ratings are effectively another language which is impossible to translate so it can be measured against established acceptable level of risk thresholds in a Risk Management Policy. Cheers Acer ventura

Lower than Low Where there is a Low risk rating there’s still scope to sell more tree work because a risk can be lower than Low. Mitigation options, irrespective of the risk rating, are part of the Risk BMP approach. Even where the risk rating is Low mitigation options are provided. A case study on p.148 of the TRAQ Manual describes a Low risk from a dead limb over a path in an urban park. The mitigation recommendation is to ‘remove the dead limb to reduce the risk’, which lowers the risk rating from Low to Low. It’s explained that if the Risk Tolerance of the client is Low there could be even more mitigation costs for them to bear by restricting access to the path until the dead limb is removed. The owner/manager is now in a position where they’re obligated to reduce a Low risk to a Low risk if they have the budget. I’ll being revisiting this case study when discussing ALARP in the next section. Cheers Acer ventura

Hobson’s Choice The only level of Risk Tolerance that can be reasonably chosen by the client/manager as acceptable is Low. A simple way of re-framing the ranges of Risk Tolerance choice in the Risk BMP. What's your Risk Tolerance for one of your children getting killed? Is it Extreme, High, Moderate, or Low? It’s not reasonably conceivable that anyone is going to have a Risk Tolerance to tree risk where they manage it at an Extreme or High level. The Risk BMP effectively excludes Extreme as an option, and the exclusion would also apply to High for obvious reasons, but also because the risk assessor is directed to ‘recommend mitigation measures’, and not provide ‘mitigation options’, with Extreme or High risk ratings. Realistically, a tree owner/manager’s choice of Risk Tolerance is limited to either a Moderate or Low risk rating from the outset. When faced with Moderate or Low risk rating there’s then little option but to go for a Low level of acceptable risk. The reason for this is partially because when faced with such a choice it's only natural to prefer a Low level of risk. However, another driver to Low Risk Tolerance is because mitigation options showing how to lower the risk from Moderate to Low are part of the Risk BMP, TRAQ training, and included on the Basic Tree Risk Assessment Form. To play out a scenario, a choice of Moderate Risk Tolerance as a Risk Management Policy has the following potential problem. After a discussion with the risk assessor about their four choices of tree risk of Severe, High, Moderate, or Low it is established the owner/manager’s Risk Tolerance is Moderate, which becomes their threshold of acceptable risk, and Tree Risk Management Policy. However, all tree reports identifying Moderate risk ratings will have mitigation options to show how the risk could have been reduced from Moderate to Low. If a Moderate risk is realised resulting in injury or death, I doubt it would be a very robust defence that the owner/manager had discharged their duty of care because their Risk Tolerance to tree risk was Moderate rather than Low. Consequently, they didn’t undertake the tree work to lower the risk from Moderate to Low, and someone got hurt or died. Cheers Acer ventura

Tolerability of Risk Framework v Risk Tolerance With QTRA the Risk of Harm from trees can be compared to levels of tolerable and acceptable risk in the Tolerability of Risk (ToR) Framework by the risk owner/manager who makes the ‘risk management’ decision. With the Risk BMP the risk owner/manager manages their acceptable level of tree risk at Extreme, High, Moderate, or Low depending on their ‘Risk Tolerance’, and this constitutes the ‘risk management’ decision. I have great difficulty squaring the QTRA approach of deferring to ToR thresholds of risk as an advisory to owners/managers, with the Risk Tolerance choices in the Risk BMP. I’m going to explore what these difficulties with Risk Tolerance are, and it should hopefully stimulate more debate because the issues are not as black and white as they were with the matrices. I’ll start off by looking what Risk Tolerance is, as outlined by the Risk BMP and TRAQ training manual.

Hi Paul Percentages might also help, and they particularly highlight the range compressions in the matrices. Assuming what I've posted previously is correct, and so far no one's suggested otherwise, on or off forums; 8 of the 16, or half, of the possible outcomes in the Risk Matrix are where there's a greater than or equal to >1/2, or 50%, Likelihood of Failure and Impact. 12 of the 16, or three quarters, of the possible outcomes in the Risk Matrix are where there's a greater than or equal to >1/4, or 75% Likelihood of Failure and Impact. A Low risk rating of death is less than <1/2. In other words, a 49% risk of death is a Low risk. A Moderate risk rating of death is less than <1/1. In other words, a 99% risk of death is a Moderate risk. A High risk rating of death occurs when the risk is greater than or >1/2. In other words, a risk of death of 49% is not considered a High risk. To compare with levels of risk in the Tolerability of Risk Framework; Unacceptable level of risk threshold = 0.1% (1/1 000) Tolerable risk threshold that can be imposed on the public = 0.01% (1/10 000) Acceptable Risk = 0.0001% (1/1 000 000) And an annual risk of death from tree failure is around 0.00001% (1/10 000 000) Cheers Acer ventura

Hi James Despite Jules' tin foil hat conspiracy theories, whether QTRA will 'have you' is absolutely nothing to do with me. As I said somewhere else' date=' Mike Ellison hired a mathematician to sort out the Monte Carlo simulations for version 5 because it was way beyond the abilities of the loose network of contributors who work behind the scenes to help develop QTRA; myself included. What I can tell you is that there are 10 000 iterations for each range combination of Target, Size, and Probability, and that the mean from a normal distribution for each combination is the Risk of Harm. If you want more information then contact Mike direct [email'][email protected][/email] IIRC you had a massive aversion to VOSL on the UKTC some years back. If you don't mind me asking, what's made you change your mind about VOSL because it's still a key component. Cheers Acer ventura

I'd forgotten about this and in effort to keep the thread on topic have replied here. http://arbtalk.co.uk/forum/general-chat/72575-quantified-tree-risk-assessment-qtra-version-5-questions-answers-2.html#post1115575 Cheers Acer ventura