410 Table 4. continued Name of method (Reference) (Country) (Coding) Private 3 (no reference) (Australia) (Private 3) QTRA – Full (Ellison 2005a, 2005b) (United Kingdom) (QTRA) Brief description and derivation of method Norris and Moore: How Tree Risk Assessment Methods Work This method based on the Australian Risk Standard matrix (AS/NZS 4360:2004) was provided confiden- tially as a simple risk matrix composed of 5 values each for the “Likelihood” and “Consequence” cate- gories. Descriptors provided guidelines for each category. The 2 scores were multiplied to produce a risk score. The risk score ranged from 1 to 25. The meaning of these scores was quantified: 1 to 6 = Low Risk, 7 to 10 = Medium Risk, 11 to 12 = Significant Risk, 15 to 20 = High Risk, > 21 = Immediate Risk. Quantified Tree Risk Assessment was a probabilistic method which multiplied 3 assessment criteria to derive an output risk score. The assessment criteria were “Probability of Failure,” “Size of Part” (Impact Potential), and “Target.” The assessor was required to quantify category data inputs, such as probability of the defect failing within the inspection period, estimated number of people passing the tree, number of vehicles per day, and size of part based on fractionising the estimated dry weight of a 600-mm-diameter tree using data derived from an allometric equation. With this method, the risk score was usually reported as the inverse of the decimal (a ratio) and termed Risk of Harm (RoH). The risk output values are expressed as a probability from 0 to 1, expressed as a ratio ranging from 1:1 to > 1:1000 billion (infinite). Acceptable risk was a RoH above 1:10,000. QTRA – Wheel Version (Ellison 2005a, 2005b) (United Kingdom) (QTRA W) Threats (Forbes-Laird 2006) (United Kingdom) (Threats) Tree Risk Evaluation (TRE) Quantitative (Standards Australia 2004a, 2004b) (Australia) (TRE QT) TRE Qualitative (Standards Australia 2004a, 2004b) (Australia) (TRE QL) US Department of Agriculture Forestry Service Method 1 (Pokorny et al. 2003) (USA) (USDAFS 1) A field tool for calculating QTRA risk levels, but values used in the various categories were not equal, and the RoH values differed from the full QTRA method. The changes effectively lowered the “Probability of Failure” input by one order of magnitude. Wheels were produced in 2005 and 2007, and this analysis was of the 2007 version, which had changes to the probability of failure range and to the vehicle use ranges within the “Target” category. A qualitative United Kingdom method, Threats used 3 assessment categories (“Likelihood of Failure,” “Target Score,” and “Impact Score”) that were multiplied to create a risk score. This method could create a zero score. The “Hazard Ratings” ranged from 0 to 20,000, and are quantified: 0 to 49 = Insignificant, 50 to 159 = Minimal, 160 to 349 = Slight, 350 to 999 = Moderate, 1000 to 2000 = Significant, 2001 to 3999 = Serious, and 4000+ = Extreme. A probabilistically based quantitative method developed for this research aligning with the Australian Standard of Risk Management (AS/NZS 4360:2004). Three categories, “Probability of Failure” (Pf), “Probability of Impact” (Pi), and “Consequences” (Co), were assigned a probability (or ratio). “Conse- quences” was fractionised based on the maximum value of a statistical life developed from data used in Australia and indexed by consumer price index. These categories were multiplied to create a risk score expressed as a probability, ratio, or a financial value. At scores below 1:10,000 risk was unacceptable; for risk scores from 1:10,000 to 1:100,000 the risk should be reduced if practicable; risk scores > 1:100,000 were considered acceptable. A method designed for quick field application, which was developed for this research, based on the same principles as TRE QT. A user interface was created with assessment categories of 5 to 7 values; the assess- ment values were summable by converting them to logarithmic scales. Risk scores from 3 to 300 were possible and could be converted to probabilities. At scores of 200 to 300 (> 1:10,000), risk was unac- ceptable; for risk scores 170 to 199 (1:10,000 to 1:100,000), risk should be reduced if practicable; scores below 170 (< 1:100,000) were considered acceptable. One of several qualitative methods developed by the USDAFS. This method summed the values from 4 assessment criteria: “Probability of Failure,” “Size of Part,” “Probability of Impact,” and “Other Risk Factors.” These categories had differing ranges, and the “Other Risk Factors” category could be used “if professional judgement suggested the need to increase the risk rating.” The output risk rating ranges from 3 to 12. No definition or quantification of the output score values was provided. ©2020 International Society of Arboriculture
November 2020
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