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

[Acer ventura]

I have my concerns about the use of Monte Carlo simulations only for QTRA, it is the single biggest turn-off for me of the whole QTRA model.

 

I told Mike what I thought about it at the Conference dinner, but after quite a few drinks maybe I didn't put the point as clearly as I thought I did. Come to think of it, he called me (with a grin) a cheeky b..... but took my point. Or so I thought.

 

Hi Jules

 

You surprise me because the use of Monte Carlo simulations in version 5 is one of the biggest turn-ons for me, and takes the credibility of QTRA to another level as a professional robust means of assessing tree risk. Are you aware of how often it’s used in risk assessments by other professions?

 

Do you want to raise your concerns about the use of Monte Carlo simulations in the QTRA v5 thread, which is gathering dust here...

 

Quantified Tree Risk Assessment (QTRA) Version 5 - Questions & Answers - Arbtalk.co.uk | Discussion Forum for Arborists

 

...so the thread can get back on TRAQ?

 

I've been busy today, so will post up the QTRA probabilities, as far as I've been able to work out, in the Risk BMP Risk Matrix tomorrow.

 

In the meantime, hopefully the better angels of James' nature will persuade him to be generous and share his explanation of where he thinks I've gone wrong in working out the Likelihood of Failure and Likelihood of Impacting the Target probabilities.

 

Cheers

 

Acer ventura

Edited May 20, 2014 by Acer ventura

[daltontrees]

Hi Jules

 

Perhaps worthy of another thread?

 

Given Nelda Matheny is one third of the principal authors of the Risk BMP, I think she might disagree with you that’s she’s misunderstood her previous work with Jim Clark.

 

I agree Matheny & Clark were pioneers with their approach in 1994. But that’s 20 years ago and things move on and develop. Moreover, there are some really important caveats in their publication about how their hazard ranking system should be applied which are still frequently ignored. One of the interesting things I’ve found researching tree risk, and running training workshops, is just how many versions of Matheny & Clark’s hazard ranking system are knocking around out there as methods of assessing risk, instead of ranking hazard. Often with additional numbers bolted on to try and massage the end result to reflect a level of risk that the designer thinks the tree should have.

 

Cheers

 

Acer ventura

 

Ah well you and she will just have to wait and see what my view on that is. Hazard assessments in isolation are about as much use as, say, shoes without feet. And without something to walk on they're both irelevant. Think fuiguratively and you'll see what I mean.

[daltontrees]

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.

[daltontrees]

Hi Jules

 

Pendant’s hat on - it would be less than <0.000061 for a Low risk rating of less than <1/2 for a fatality. But that would only be the case if you pick the 14 out of the 30 trees that all have an equal Low risk rating of less than <1/2 of failing and causing a fatality, AND then identify the correct sequence in which each one of them fail and cause a fatality over the year. What you’ve done is calculate the odds of picking the correct sequence of heads or tails with 14 consecutive tosses from an unbiased coin.

 

Cheers

 

Acer ventura

 

Nope.Each tree either fails or ti doesn't. The sequence is irrelevant. If tree 1 fails and tree 2 fails and tree 4 fails and tree 7 fails etc. te probability of each is 1/2 and the probability of any 14 failures is 1/2 x 1/2 x 1/2 etc. 14 times, which is 0.000061. But since you were setting out initially to express the risk of this in 'language outside of probabilities', it would suffice to express it as freakishly bad luck. Like a coin toss coming up tails evey time 14 times in a row, it's unlikely but in practice and in theory it's possible.

[daltontrees]

Hi Jules

 

The 'Risk of Harm' at the top of the slide are my words, not NASA’s which are in quotation marks, because the slide is in the context of a QTRA presentation and that’s how QTRA refers to the risk probability. Nicholas Johnson referred to a ‘human casualty’, which is a euphemism for a fatality. I suspect part of NASA's model is that if someone is unfortunate enough to be hit by a bit of falling satellite they're likely to die. If you look online, at their website and articles about what the Orbital Debris researchers have been looking to achieve, it seems clear the consequences are a fatality.

 

Cheers

 

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.

[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.

 

My apologies for the delay because I’m now going to post the QTRA populated BMP Risk Matrix tomorrow morning. Discussing the Consequences input to the Risk Matrix, relative to the QTRA inputs and Risk of Harm outputs, means the accompanying text is lengthier than I hoped it would be. I’m sure it will benefit from a night’s sleep, some editing, and my near hapless ability to effectively proof read out the inevitable word blindness over very short time periods (though I quite like the irony of a pendant hat).

 

Jules, I will reply to some of your posts. However, I’m going to do so on the QTRA v 5 thread in an effort to prevent this thread from being driven into peculiar backwoods cul-de-sac where feet, footwear, and the possible lack of them, are not significant in either a figurative or standardly deviant way. In the meantime, see whether you can get your head around the difference between ‘independent’ and ‘dependent’ probabilities and why you don’t cumulatively multiply the former.

 

Cheers

 

Acer ventura

[Acer ventura]

Here’s the BMP Risk Matrix, with the cells completed for their respective risk ratings, but coloured and populated according to QTRA advisory thresholds (second slide) and the Risks of Harm, as probabilities, for damage to property only.

 

The QTRA Target Ranges for property are labelled ‘T1’ etc, in the Consequences column titles, and were derived from the descriptions of damage to property in the Risk BMP and TRAQ Manual.

 

T1 = £1 500 000

T2 = £150 000 - >£15 000

T3 = £15 000 - >£1 500

T4 = £1 500 - >£150

T5 = £150 - >£15

T6 = £15 - £1

 

 

 

 

This is a tentative categorisation because the monetary damages in the Risk BMP are not clearly defined and ambiguous. They are described as;

 

Negligible = Low monetary damage

Minor = Moderate monetary damage

Significant = High monetary damage

Severe = There is no monetary damage descriptor

 

There is no indication of what Low, Moderate, or High monetary damage are in terms of actual monetary value.

 

There are other descriptors to give some clues;

 

Negligible

“A small branch striking a fence.”

“A medium-sized branch striking a shrub bed”

“A large part striking a structure and causing low monetary damage”

 

Minor

“A small branch striking a house roof from a high height.”

“A medium-sized branch striking a deck from a moderate height.”

“A large part striking a structure and causing moderate monetary damage.”

 

Significant

“A medium-sized part striking an unoccupied new vehicle from a moderate or high height”

“A large part striking a structure and resulting in high monetary damage”

 

Severe

No monetary damage descriptor.

 

It’s not clear;

 

At what diameter and/or length a ‘small branch’ becomes a ‘medium-sized branch’, or a ‘medium-sized’ branch becomes a ‘large part’.

 

Where a ‘low height’ becomes a ‘moderate height’, or a ‘moderate height’ becomes a ‘high height’.

 

Or, to play out one example, whether a medium-sized part striking an unoccupied new vehicle from a ‘low’ height means the Consequences are now Minor with a ‘moderate monetary damage’ rather than Significant with ‘high monetary damage’.

 

Or, how older a vehicle has to be than a new one in the Significant example, before the Consequences of being struck by a medium-sized part from a moderate or high height go from Significant high monetary value down to Minor moderate monetary value.

 

And so forth.

 

As you can see from the matrix, the QTRA Risks of Harm are the same for all the Likelihood of Failure and Impact input ranges of Very Likely, Likely, Somewhat likely, and Unlikely (the upper values for Unlikely) within each Consequence column. This is because all the Likelihood of Failure and Impact input ranges (the upper values of Unlikely range) are all extremely compressed into one QTRA Probability of Failure Range 1 (1/1 – 1/10).

 

So, the 'Severe' Consequences risk ratings column of Extreme, High, Moderate, and Low are all red unacceptable risks at 1/3 with Low being less than <1/3.

 

The 'Significant' Consequences risk ratings column of High, Moderate, and Low are all red unacceptable risks at 1/300 with Low being less than <1/300.

 

The 'Minor' Consequences risk ratings column of Moderate and Low are all amber unacceptable risks to impose (unless there is great value or the stakeholders agree) at 1/3 000 with Low being less than <1/ 3 000.

 

The 'Negligible' Consequences risk ratings column are all Low, and are yellow risks at 1/30 000, except the 'Unlikely' cell which is <1/30 000, which are tolerable risks that can be imposed if the risk is ALARP.

 

There is considerable compression in the Severe Consequences range because (T1 – T2) £1 500 000 - >£15 000, of the QTRA property damage range, falls within Severe.

 

Any suggestions as to where the QTRA Target Ranges might be any different for Negligible, Minor, Significant, and Severe Consequences to property?

 

One thing I haven’t been able to figure out is how or why the “disruption of arterial traffic or a motorways” is in the same Consequences range, Severe, as a death.

 

Cheers

 

Acer ventura

[daltontrees]

. In the meantime, see whether you can get your head around the difference between ‘independent’ and ‘dependent’ probabilities and why you don’t cumulatively multiply the former.

 

Cheers

 

Acer ventura

 

Is that meant to be humour? The independent failure at 50:50 of a number of trees is independent probability. The probability of all of them failing can only be obtained by multiplying the individual probabilities. That is damn near the definition of the multiplication theorem. Please don't unnecessarily cast a shadow of discreditability over the othehrwise interesting and thorough thread by suggesting that one of the very few commentators (me) is wrong on something like this when I am patently right. Not because I have any particular insight, it's just because the scenario is no more complex than repeated coin-tossing.

[Acer ventura]

Is that meant to be humour? The independent failure at 50:50 of a number of trees is independent probability. The probability of all of them failing can only be obtained by multiplying the individual probabilities. That is damn near the definition of the multiplication theorem. Please don't unnecessarily cast a shadow of discreditability over the othehrwise interesting and thorough thread by suggesting that one of the very few commentators (me) is wrong on something like this when I am patently right. Not because I have any particular insight, it's just because the scenario is no more complex than repeated coin-tossing.

 

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

[treeseer]

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.

 

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.