Tree Risk Assessment Qualification (TRAQ) – ISA Best Management Practices

[daltontrees]

Hi Jules

 

I think everyone else is likely to be pretty bored of this sub-thread, and it's way off topic. I've transplanted it to the QTRA v5 thread, as I will do my reply to you about Monte Carlo simulations, and what you think the Risk of Harm means, when I get the time.

 

I've replied to you here.

 

http://arbtalk.co.uk/forum/general-chat/72575-quantified-tree-risk-assessment-qtra-version-5-questions-answers.html#post1114407

 

Cheers

 

Acer ventura

 

I ight wander over to the QTRA thread sometime. Meanrime all I would say is that my coememtns aren't particularly off-thread, if anything I am trying to apply a bit of peer-review to your conmclusions because I canot accept and am reluctant to let anyone else accept your preliminary conclusions about the schoolchildren scenario and what this says about flaws in the logic of the thread as a whole. There is always a huge huge problem presenting probabilities in numbers or words as facts because the scenario has rarely been clearly and rigorously defined. For example, if my calculations relate to 14 nominated trees killing a child, I might be right. If they relate to any 14 of teh 30 my figures are inappropriate.

 

If anyone cares, the point is this.You would need to set up a hypothesis, let us call it the Tree of Damocles hypothesis. 30 children stand beneath 30 trees for the entire period between inspecvtions, one beneath each tree. Only one child will be killed by each tree if it fails. Furthermore, if it fails it will only kill, not maim, injure, frighten or narrowly miss. To be even more specific, if it fails the only outcome will be to kill a child. Improbable in common language but this is a thought experiment and no children will be harmed in the making of this hypothesis.

 

Nominate 14 trees and I'm right. State that it can be any 14 of the 30 trees, and my answer is inappropriate and the corrected answer will be along shortly, after I have had my dinner and voted int eh Eurpoean Elections. I fi don't fall asleep first. I have been surveying (oops, risk assessing) trees all day and I am done it.

[Acer ventura]

TRAQ assessments ime most oft go astray when 'Possible' is ticked when a likelihood is quite Improbable, because 'Anything is Possible', or so they say.

 

This semantic snafu results in no Unlikely, and no Low Risk, even when risk is Very low.

 

Hi Guy

 

A point well made about a risk assessors own definitions of what Possible means to them overriding guidance. For me, Possible is the most problematic descriptor in the Likelihood of Failure ratings because “Failure could occur, but is unlikely” covers everything apart from Imminent. I’m well aware that part of using the Risk BMP is that you buy into their definitions of what Possible means relative to other Likelihood of Failure descriptors, but boy does it cause some cognitive dissonance.

 

However, are you sure you've got the Possible = No Low Risk conclusion right? According to the Likelihood Matrix, when the Likelihood of Failure is Possible you can get an Unlikely with Medium, Low, and Very low Likelihood of Impacting the Target.

 

 

In the Risk Matrix an Unlikely Likelihood of Failure & Impact = Low Risk rating for all Consequences.

 

 

Too many matrices; brains are tired when they get to Mitigation Options, so the most important considerations are typically ignored. As with the 1994 form, 'Fell' is all too often the result.

 

Something I’ve not figured out yet, and so far found nothing published, is reference to risks being assessed and rated using two matrices. I’ve not encountered this double matrix approach before, and it has occurred to me the intrinsic limitations of using risk matrices might be especially compounded where two are used and the outputs of one are fed into the other. For example, the lack of exclusivity and significant overlap in the Likelihood of Failure and Impact ranges in the Risk Matrix are the primary reason that qualitative lower risk ratings can be quantitatively higher than the qualitative higher risk rating above them.

 

 

Does anyone know of any double matrix risk assessments?

 

Cheers

 

Acer ventura

[Acer ventura]

There’s not been that much traffic in the way of contributions to this thread so far (more off forum), but clearly there’s a lot of interest judging by the amount of views after a new post. The lack of contributions doesn’t bother me because the way I figure it, this being the interweb, and a discussion forum for arborists, if I’ve got something wrong, there will be at the very least someone eager to point this out. And what we’ve gone through so far is largely mathematical and binary.

 

I’ve reached a crossroads here and I’m not sure which section to develop next. The purpose of the thread is to explore the possibilities of QTRA and the Risk BMP working in tandem, by analysing the Risk BMP for common ground. When I first started to look at this I thought the main points of potential conflict were going to be the ‘risk tolerance’ issue, and that there’s no scope for a risk to be As Low As Reasonably Practicable (ALARP) with the Risk BMP. That was before I started dissecting the matrices. Looking at the matrices in detail was an interesting exercise and there are more issues with the matrices than I’ve gone through so far - which might be the underlying reasons to explain why the output risk ratings look like they could be untenable - and I was going to explain what they are.

 

Upon reflection, I’ll go with what I think is the more important issue, which is ‘risk tolerance’, on Monday. This is less of a yes/no binary subject and will hopefully generate a bit more forum debate.

 

As ever, any comments, on or off forum, are very welcome.

 

Cheers

 

Acer ventura

[Paul Barton]

Well, as usual I have got lost in this discussion as my brain started to swim with all the numbers. Perhaps it's just me:blushing:

 

One thing I have noticed: all the workings out that Acer has done to replace the descriptive words used in TRAQ with a numerical value start on the premise that:

• Imminent = 1/1
  
• Severe = 1/1
  

 

But "imminent" does not mean "certain" or "definite". Perhaps it means something more like "highly probable" or "almost certain". Either way, I don't think you can equate it with 1/1 (i.e. 100% probability).

 

Perhaps the upper threshold of 1/1 is too high, and that has affected all the other values which Acer has suggested - thus making TRAQ seem more risk averse than the QTRA method.

 

It would be good to have some wider contributions to this thread as currently it is being dominated by two people that clearly have abnormally large brains. After all, if these risk assessment methods are to be used by a large number of arborists, they need to be simple enough for the average person like me to understand:001_cool:

[Craig Johnson]

you can put names or numbers into a system, but its all just guessing, as we cant say that .... will fail on Tuesday at .....

I don't think we could even get the year right;-)

[Acer ventura]

Well, as usual I have got lost in this discussion as my brain started to swim with all the numbers. Perhaps it's just me:blushing:

 

Hi Paul

 

Initially, it can take some time to get your head around numbers if you’re not used to working with them. Doing the research into this thread, I spent a long time scratching my head during the working out, discussing it with others, and checking it over and over. It’s worth a re-visit after some digestion, taking things in bit-by-bit because what the numbers do is remove the subjectivity and ambiguity. What they give is objectivity and clarity.

 

Asking questions, like you’ve done here, is also a good way of getting your head around things. I tend to the view, there's no such thing as a dumb question and would be great to have some more. All I've done here is outline what I think I've worked out. I could be wrong, and I would welcome anyone posting to explain where and why I've gone wrong.

 

One of the reasons I decided to set this thread up the way I have is so it will be there, step by step, and completely transparent, for anyone to go through and agree or disagree with.

 

One thing I have noticed: all the workings out that Acer has done to replace the descriptive words used in TRAQ with a numerical value start on the premise that:

 

Imminent = 1/1

Severe = 1/1

 

But "imminent" does not mean "certain" or "definite". Perhaps it means something more like "highly probable" or "almost certain". Either way' date=' I don't think you can equate it with 1/1 (i.e. 100% probability).

 

Perhaps the upper threshold of 1/1 is too high, and that has affected all the other values which Acer has suggested - thus making TRAQ seem more risk averse than the QTRA method.[/quote']

 

Here’s the explanation fleshed out a bit more from what I put together in the earlier posts.

 

Imminent

From the Risk BMP. “Failure has started or is most likely to occur in the near future, even if there’s no significant wind event or increased load. This is an infrequent occurrence for a risk assessor to encounter and may require immediate action to protect people from harm.”

 

Within the time frame of a year, that’s easily got to be 1/1.

 

Also, Imminent is the highest of the Likelihood of Failure ranges and therefore its highest value has to be the top value of all four ranges, which is 1/1, or 100% if you prefer. A further consideration is that if it isn't 1/1 then there’s a gap at the top of Likelihood of Failure because the highest value isn’t accounted for.

 

Imminent is curious because from the Likelihood of Failure description it is a fixed point, and not a range with boundaries defined by upper and lower values.

 

Furthermore, Imminent cannot be anything but 1/1 because the next range down in Likelihood of Failure is Probable, which can only reasonably have a lowest value in its range of greater than >1/2 (more likely than not), and a highest value in the range of 1/1 for the timescale of a year.

 

Severe

Similarly, Severe is the highest Consequence so it has to have the highest value of all four ranges, and the highest value would be the loss of a life. Therefore Severe = 1/1.

 

Something to bear in mind with the Risk BMP is that it’s a qualitative risk assessment system, so it’s from the contents of that publication, and the TRAQ training manual, that you have to go to get the definitions of what a matrix word actually means. It’s not what you or I think a word means. It’s a bit like Humpty Dumpty in Lewis Carroll’s ‘Through the Looking Glass’, when he says;

 

“When I use a word….it means just what I choose it to mean — neither more nor less.”

 

Sometimes it’s clear what the words mean by thinking about them, as discussed above. The highest values for both top Likelihood ranges of Imminent and High has to be 1/1, and highest value for the Consequences range of Severe has to be 1/1.

 

Or when a Medium Likelihood of Impacting the Target is described as “The failed tree or part is as likely to impact the target as not.” It’s clear that this is 1/2 or 50%.

 

Conversely, you get to the lower two Likelihood of Failure ranges of Possible and Improbable. Possible has no clues as to what the lowest value of its range could be, and similarly, Improbable has no clues as to what the highest value of its range could be. Which is why I've labelled these '?'

 

Does that help?

 

Cheers

 

Acer ventura

[Acer ventura]

you can put names or numbers into a system, but its all just guessing, as we cant say that .... will fail on Tuesday at .....

I don't think we could even get the year right;-)

 

Hi Craig

 

And that’s one of the beauties of using probability to estimate the likelihood of an uncertain event. You’re not saying it will, or will not happen. You’re estimating the odds of it happening. One of characteristic of probabilities that many don’t immediately grasp, when you use them in a tree risk assessment, is that you’re conveying your uncertainty about what might happen in the event of a tree or branch failing and hitting someone or something.

 

There’s a really neat example that Douglas Hubbard uses which illustrates this, and also neatly nails unfounded criticisms about the use of probabilistic numbers implying false precision. It’s from Chapter 7 - ‘Worse Than Useless; The Most Popular Risk Assessment Method and Why it Doesn’t Work’, in ‘The Failure of Risk Management: Why it's Broken and How to Fix it’. It's a great book, and is part of the foundations on which this thread is built.

 

If you have no idea whether it’s going to rain tomorrow you could express this likelihood with, ‘there’s 50% chance of rain tomorrow’. 50% looks like a very precise figure but what it means is that you’re expressing the highest level of certainty that you can about whether it will rain tomorrow. There isn't a higher level of uncertainty.

 

Cheers

 

Acer ventura

[Acer ventura]

I have nothing further to say to QTRA about Monte Carlo simulations. They do little more meaningful than standard deviations can do, unless you assume (unintuitively, in my opinion) that risk is preceived on a linear scale.

 

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#post1115583

 

Cheers

 

Acer ventura

Edited May 24, 2014 by Acer ventura

[Acer ventura]

Ah but QTRA expresses relative damage or harm as a proportion of the value of a statistical life, does it not, and if so everything should be related to that standard and the term 'harm' is I would suggest (strictly speaking) inappropriate.

 

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

[Paul Barton]

 

Does that help?

 

Cheers

 

Acer ventura

 

It does, thanks.