412 Norris and Moore: How Tree Risk Assessment Methods Work Table 5. Qualitative risk score scaling assigned to tree risk assessment methods to allow comparison of the different outputs and risk ratings for use in Minitab™. Risk scale rank Risk descriptor Bartlett CTC HCC MandC Kenyon Private 1 Private 2 Private 3 QTRA QTRA W Threats TRE QT TRE QL USDAFS 1 USDAFS 2 TRAQ 1 2 3 4 5 6 2 1 3 - < $100 1 - - > 1:1m > 1:1m > 50 - - - 0 1 3 - - - - - 5 1 - - 159 - - - 1 2 5 - - 4 - 125 10 6 - - 160 > 1:100k > 1:100k 3 2 3 6 14 - - - 250 - - < 1:10k < 1:10k 349 - - - 3 4 - 15 - - $10,000 - 10 7 - - 350 - - - - 6 moderate 7 - - 7 - 375 - - - - - - - 7 4 8 7 8 9 35 - - - - 16 10 - - 999 - - - - - high 10 36 - - - - - 12 - - 1000 < 1:10k < 1:10k - - 9 9 10 Insignificant Very low Low Minor/ Median Medium/ Moderate Somewhat High Extreme slight 13 60 64 12 - 375 20 15 - - 2000 - - 12 6 12 15 N/A N/A N/A > $1m 500 24 21 N/A N/A 4000 N/A N/A N/A N/A 16 Note. For some methods, the highest risk rating was extreme and for others high. For clarity and simplicity, this table only shows method scores that directly match the 10 point scale and/or the verbal descriptor of risk. Consequently, individual input analysis for meth- ods such as MandC, Private 2, QTRA, and TRE QT identified that each input produced the same influ- ence on the output value. Probabilistic methods recorded a percentage change that reflected the input change, where a 25% change to an input modified the output value by 25%. For qualitative methods using ordinal ranks such as MandC, Private 1, and Private 2, the percentage change to the output values varied sig- nificantly. For MandC, a 1 rank change from the mean modified the output value by 17%; for Private 2, the change was 8%. Private 3 recorded a 33% change to the output value, and for TRAQ, a 1 rank change to impact or failure moved the risk rating 1 step, but a change to consequences did not alter the risk rating. Of the methods reviewed, 8 were designed such that each input category exhibited a different influ- ence on the their respective output value, and due to category scaling differences, a 1 rank change in dif- ferent directions could create a large change in the output values. For example, QTRA W, due to its log- arithmic nature, exhibited extreme intra-category variation (a + 1 rank change increased the output by 900%, whilst a 1 rank decrease reduced the output value by 81%). Methods such as USDAFS 2, Threats, ©2020 International Society of Arboriculture and Kenyon exhibit very large movements in both inter- and intra-category values for simple 1 rank changes (Table 6). The 1 rank or 25% change sensitivity analysis clearly and simply demonstrated that different risk assess- ment methods approached the input weighting and scaling differently and combined them mathemati- cally differently. Hence, a change to 1 input value could create a wide range of changes to the output values and hence risk rating. Only the QTRA and the 2 TRE methods provided an explanation for the range and scaling of inputs, weighting of categories, or combi- nation mathematics, which makes working with other methods more difficult and less certain. The Monte Carlo simulation for the 16 methods aligned with the simpler rank-change analysis (Table 6). Simple linear models that sum the inputs, such as MandC, TRE QL, USDAFS 1, and Private 2, pro- duced distributions that approximated normal distri- bution curves, with the majority of outputs towards the centre of the distribution. The 2 QTRA approaches and TRE QT are mathematically probabilistic and hence produce logarithmic distributions. These methods produced output scores over a wide and sometimes extreme range, where, for example, the lowest value
November 2020
| Title Name |
Pages |
Delete |
Url |
| Empty |
Search Text Block
Page #page_num
#doc_title
Hi $receivername|$receiveremail,
$sendername|$senderemail wrote these comments for you:
$message
$sendername|$senderemail would like for you to view the following digital edition.
Please click on the page below to be directed to the digital edition:
$thumbnail$pagenum
$link$pagenum
Your form submission was a success. You will be contacted by Washington Gas with follow-up information regarding your request.
This process might take longer please wait
