Quantified Tree Risk Assessment (QTRA) - Questions & Answers

[Acer ventura]

But who's to say a tree with an outcome of 1/10050 is safer than a tree with 1/9997...?

 

It's this variability that I can't seen to get past. Life just isn't exact?

 

Hi Robin

 

And neither is QTRA 'exact', nor does it claim to be. It's a 'reasonably practicable' and 'proportionate'* risk assessment. You can't get "an outcome of 1/10050 is safer than a tree with 1/9997" with QTRA. I thought I'd covered this in the previous post, so I'll expand and try with a different tack. Paradoxically, perhaps it's all the effort that has been made to take 'variability' by the hand and be inexact that has made you think that QTRA is trying to be unreasonably exact.

 

Let's roll up our sleeves and delve into the nuts and bolts of the QTRA engine. This is dirty under the bonnet stuff poking around in the cylinder head of underlying principles, which it is not necessary to know in order to drive QTRA (line up three ranges), but may help you understand what is going on.

 

RoH is not a measure of 'exactness' or 'precision', and neither is the PoF. It's a probabilistic risk assessment and therefore an expression of 'uncertainty', or 'inexactness', if you like. Through probability calculated from the highest value of the broad ranges, we can operate within the realms of 'reasonable practicability' and 'proportionality'.

 

The element of the QTRA calculation that has the greatest level of uncertainty is the PoF. You may have noticed in the example I gave previously, unlike the Target and Impact Potential, there was no scope to refine the PoF range. We are particularly mindful of uncertainty, or inexactness, in PoF which is why in QTRA guidance PoF has broad ranges and there's no scope to refine it because with current knowledge we don't think you can do this with a greater level of certainty. To try and refine PoF is not 'reasonably practicable' or 'proportionate'. In the example I gave, PoF falls within a range at some point between 1/100 -1/900, so we're up front with the fact we're not sure, we're uncertain, we're inexact and therefore to be on the safe side it is the highest value of 1/100 in the range that is going into the risk calculation.

 

That RoH is expressed to 1 significant figure only is an extension of this uncertainty. RoH is a worst case scenario expression because it is the product of the highest value of the three input ranges, and in the example I used in the previous post where we get;

 

RoH = 1/10,000 (T = 3 x IP = 2 x PoF = 3).

 

However, if you calculate the actual probabilities

 

T= 1/72 x IP= 1/2 x PoH= 1/100 = RoH 14,400

 

Because PoF is expressed to 1 significant figure, then so is the RoH, and 14/400 is therefore expressed as 1/10,000.

 

What we've done with QTRA is put a lot of effort into embracing 'uncertainty', and worked really hard at not being 'exact', because that's what you do when you’re being reasonably practicable and proportionate and quantifying risk with imperfect knowledge.

 

Does that help?

 

Cheers Acer ventura

 

*'Reasonably practicable' and 'proportionate' means;

 

Doing only what is necessary and reasonable in the given circumstances.

Considering the benefits derived from the hazard as well as the risks.

Expending resources only at a level that is proportionate to the projected reduction in risk.

Reducing risk to a reasonable level but not necessarily eliminating it.

Balancing Costs with Benefits.

[Acer ventura]

I recently posted a variation of this on the UKTC's ‘Defect led Tree Risk Assessment - let's change the title! Numbers and Words’ thread, and given where we are with this thread about ‘exactness’ and ‘uncertainty’, it seems appropriate to raise it here.

 

Currently, QTRA is undergoing the next step in its development to better encapsulate the ‘uncertainty’ element of risk. We’ve adopted a tame mathematician and got them running Monte Carlo Simulations on thousands and thousands of possible outcomes within the three input ranges at particular confidence levels to generate probability distributions and their respective averages. Those average outputs can then be related to the three thresholds in the 'Tolerability of Risk Framework (ToR)' (1/1,000 - 1/10,000 - 1/1,000,000) – though we may have 4 to include 1/100,000. What all this means is we’ll end up with a RoH that better express the uncertainty in the risk assessment, rather than specific numerical probabilities. The result of this evolution is that QTRA will be probabilistically more robust, easier to use for the risk assessor, and easier to interpret for the risk manager.

 

For those of you who are interested in what on earth this Monte Carlo jibber-jabber is all about, here's an easy to grasp short visual example about it which is pretty good. Well, pretty good until Kevin strays out of his field of expertise and drifts into European geography.

 

Cheers

 

Acer ventura

 

[ame=http://www.youtube.com/watch?v=Xaymy3Blnq4]Episode 2: Monte Carlo Simulation - YouTube[/ame]

[RobArb]

Thank you David for taking the time to reply, sorry i haven't replied back as yet, been a busy boy today with assignments and want to sit and divulge this information properly.

 

I have heard of the Monte Carlo Simulation before, its vaguely linked into the Wisdom of Crowds. I was never great at Maths at school, but never really put the effort into what it deserves, but that's a whole different story!

 

 

 

 

On a quick note before I reply properly, You bring in the term 'Reasonably practicable' or "ALARP" or "SFAIRP", but i always figured QTRA advertised itself as something different from the current forms of tree hazard assessment. Deciding whether a risk is ALARP can be challenging because it requires duty-holders and the experts advising them to exercise judgement (there's that subjectivity again). In the great majority of cases, that judgment could refer to existing ‘good practice’ that has been established by a process of discussion with stakeholders to achieve a consensus about what is ALARP. For high hazards, complex or novel situations, good practice is built upon, using more formal decision making techniques, including cost-benefit analysis, to inform judgement. But why should i use QTRA over what already exists...?

Edited February 9, 2013 by RobArb

[Acer ventura]

Thank you David for taking the time to reply, sorry i haven't replied back as yet, been a busy boy today with assignments and want to sit and divulge this information properly.

 

Hi Robin

 

There's no time constraints on exchanges here. In these days of instant communication it's all too easy to let the fingers reply to a message before the brain has had a chance to intervene. Taking the time to have a think about something is a good thing.

 

I have heard of the Monte Carlo Simulation before' date=' its vaguely linked into the Wisdom of Crowds.[/quote']

 

The Wisdom of Dice would be a better simile. Not in a Luke Reinhart sense, but in that they don’t have an opinion.

 

On a quick note before I reply properly' date=' You bring in the term 'Reasonably practicable' or "ALARP" or "SFAIRP", [b']but i always figured QTRA advertised itself as something different from the current forms of tree hazard assessment[/b]. Deciding whether a risk is ALARP can be challenging because it requires duty-holders and the experts advising them to exercise judgement (there's that subjectivity again). In the great majority of cases, that judgment could refer to existing ‘good practice’ that has been established by a process of discussion with stakeholders to achieve a consensus about what is ALARP. For high hazards, complex or novel situations, good practice is built upon, using more formal decision making techniques, including cost-benefit analysis, to inform judgement. But why should i use QTRA over what already exists...?

 

I didn't pick this bit up in the email alert, and I see you've edited to add it, so I'll get back to you on it later (depending on how refreshed I get during the Ireland v England game this afternoon). However, there's one point I can make straight away. QTRA is a risk assessment, not a hazard assessment. This isn't semantic pedantry, it's a really important distinction.

 

As a teaser, if you quantify risk with QTRA, determining whether a risk that falls within the ToR Framework Tolerable Region between 1/10,000 - 1/1,000,000 is ALARP is very straight forward.

 

Cheers

 

Acer ventura

[daltontrees]

Hello I have just come across this thread, catching up on it has been hard work.

I advocated quantification of tree risk in the original RobArb thread but I suspect I failed to persuade. By happy coincidence the Ireland England game has created a lull at the juncture where my only principal quaestion in the whole matter can be inserted just when it is about to be discussed.

Any system that you have to pay for but is a 'black box' i.e. input plus some mysterious process equals output is liable to be viewed cyncially when a more open and free system is available. Hopefully it will be clear from the other thread that I am obsessively in favour of attempting to quantify risk.

The question; appreciating that you may have a slightly vested interest in arguing one way rather than the other, nevertheless can you say whether a satisfactory risk assessment can be achieved in general circumstances without the use of the paid-for QTRA model? Your persuasive reasoning thus far seems to suggest that it can by the average competent assessor.

[Acer ventura]

On a quick note before I reply properly, You bring in the term 'Reasonably practicable' or "ALARP" or "SFAIRP", but i always figured QTRA advertised itself as something different from the current forms of tree hazard assessment.

 

Hi Robin

 

I think you meant 'risk' rather than 'hazard', but for anyone who might be following this and isn't sure of the distinction, and so we're singing from the same hymn sheet.

 

Hazard

“A hazard is the disposition of a thing, a condition or a situation to produce injury.” (HSE)

 

Risk

The probability of something adverse happening.

 

Every tree is hazardous to some extent, and extremely hazardous trees can have tolerable or acceptable levels of risk.

 

As an aside, hazards can have substantial ecological value. Something that’s really promoted on this site from those interested in veteran trees. I think it was David Lonsdale who was telling me that the Forestry Commission’s ‘Recognition of Hazardous Trees' leaflet was withdrawn because of its similarity to the ideal veteran tree (Veteran Trees: A Guide to Good Management).

 

 

 

Deciding whether a risk is ALARP can be challenging because it requires duty-holders and the experts advising them to exercise judgement (there's that subjectivity again). In the great majority of cases' date=' that judgment could refer to existing ‘good practice’ that has been established by a process of discussion with stakeholders to achieve a consensus about what is ALARP. For high hazards, complex or novel situations, good practice is built upon, using more formal decision making techniques, including cost-benefit analysis, to inform judgement.[/quote']

 

I know.

 

Risk management: ALARP at a glance

 

I’m not aware of any other tree risk assessment methodology, apart from QTRA, that even considers ALARP, so I'm not sure what you mean by "not different" in your first sentence. The National Tree Safety Group (NTSG) make considerable reference to it but have no suggestion for mechanisms to work it out.

 

Have a read through the Practice Note. The issue of ALARP and ‘gross disproportion’ is dealt with a number of times in Section 2 in Definition of Terms under ‘ALARP’, ‘Acceptable and Tolerable Risks’, and there’s an example of calculating whether a tree risk is ALARP in ‘Example 3’ on page 8. It is seldom the case that a tree with a RoH of less than 1/10,000 is not ALARP.

 

Quantified Tree Risk Assessment

 

But why should i use QTRA over what already exists...?

 

The first part of the answer is in my first post on the thread' date=' and here's one I made earlier.

 

Why Use QTRA.pdf

 

Cheers

 

Acer ventura

[RobArb]

I can't view that document on my phone?

 

Shall have another good read when I'm home. I must say you do like to make a good argument:thumbup:

 

Sent using Arbtalk Mobile App

[RobArb]

Any system that you have to pay for but is a 'black box' i.e. input plus some mysterious process equals output is liable to be viewed cyncially when a more open and free system is available. Hopefully it will be clear from the other thread that I am obsessively in favour of attempting to quantify risk.

The question; appreciating that you may have a slightly vested interest in arguing one way rather than the other, nevertheless can you say whether a satisfactory risk assessment can be achieved in general circumstances without the use of the paid-for QTRA model? Your persuasive reasoning thus far seems to suggest that it can by the average competent assessor.

 

Also, would just like to reiterate Dalton trees post as I feel it's a good question:biggrin:

 

Sent using Arbtalk Mobile App

[Acer ventura]

I advocated quantification of tree risk in the original RobArb thread but I suspect I failed to persuade.

 

Hi Jules

 

Thanks for chipping in, and good to hear you get the value of quantifying tree risk. You may particularly enjoy the quote at the head of the QTRA Practice Note.

 

"When you can measure what you are speaking about, and express it in numbers, you know something about it;

but when you cannot measure it, when you cannot express it in numbers, your knowledge is of a meagre and unsatisfactory kind”

 

William Thomson, Lord Kelvin, Popular Lectures and Addresses [1891-1894]

 

Any system that you have to pay for but is a 'black box' i.e. input plus some mysterious process equals output is liable to be viewed cyncially when a more open and free system is available. Hopefully it will be clear from the other thread that I am obsessively in favour of attempting to quantify risk.

 

On the contrary' date=' there’s nothing at all [b']‘black box’ [/b]about QTRA. Just the opposite is the case. Absolutely everything about how each component is derived, and how the risk is calculated, is transparently laid out in great detail in the Practice Note (you don’t need to be a registered user to see this) and User Manual. There’s a link to it in my reply to Robin.

 

Of the free systems that are available, I don’t know of any that aren’t built on ‘black boxes’ and 'mysterious processes' in that the hows and whys of the components of the risk are not disclosed. They just are. Or where a level of acceptable risk is suggested, the basis for it is not disclosed; ie it’s little more than the opinion of the person that put it together.

 

The question; appreciating that you may have a slightly vested interest in arguing one way rather than the other' date=' nevertheless can you say whether a satisfactory risk assessment can be achieved in general circumstances without the use of the paid-for QTRA model? Your persuasive reasoning thus far seems to suggest that it can by the average competent assessor.[/quote']

 

I do have a limited vested interest in that I get some coin when I run workshops, but I don’t take a salary from QTRA. If I came across a tree risk assessment system that was better than QTRA then I would use that. In such circumstances, I suspect Mike Ellison would also wind up the company. If there was some way of financing and developing QTRA without registered users paying for the training and an annual fee, I’m sure Mike would jump at the chance. IIRC he offered it the ISA, and informally sounded out the AA about them taking it on, but neither did.

 

I think there are considerable limitations to undertaking tree risk assessments without using the QTRA model, and enormous benefits to the risk assessor and risk manager in using it (see my 'here's one I made earlier pdf slide to Robin). For starters, I don’t know how else to quantify risk relative to the 1/10,000 tolerable level of risk, or the 1/1,000,000 broadly acceptable level of risk, without using QTRA.

 

As for the limitations. Without a quantifiable level of tolerable or acceptable level of risk being determined, risk management decisions are placed entirely with the tree risk assessor. This management responsibility can result in a natural tendency for them to take a defensive ‘risk averse’, rather than ‘risk aware’, approach when assessing trees. Consequently, the focus can all too easily be on a ‘what might happen’ rather than ‘what is likely to happen’, that often results in efforts to minimise or remove all but extraordinarily low levels of risk, rather than manage risk to a reasonable level.

 

Cheers

 

Acer ventura

[daltontrees]

Since Lord Kelvin's name has been evoked, let me indulge myself with a cheesy link that a Radio 2 DJ would be proud of. I am currently surveying all the larger trees on the north side of the Kelvin for the local authority. You may (if you know the area) think 'NO! Surely not? That's a ridiculously onerous and enormous task.' We ll it is enormous and it is onerous but it will hopefully turn out by the end of March to have been possible. Not only am I to carry out a VTA of the trees but I have to assess the risk relative to the 1: 10,000 and 1:1,000,000 thresholds and in the band between I must specify and prioritise the treeworks necessary to achieve the ALARP criteria.

 

I don't use any black box system. I am there on survey for several weeks, and being from the area generally I am aware of the way that the area is used habitually and occasionally by residents and visitors. The other day I found myself faced with a medium sized tree with high probability of failure (it definitely won't be there in 5 years) and will kill someone outright if they are unfortunate enough to be underneath when it fails. The usage of the path is low but I know that in summer there will be people passing and lingering frequently. A quick calculation, with chalk on the pavement, suggests that if there is someone beneath the tree for only 1 minute a year the risk is unacceptable at 1:1,000,000 and on the ALARP principle with my estimation of the average at-risk time throughout the year, the risk cannot be rendered acceptable except by felling the tree within 3 months.

In the very woods that Lord Kelvin would have seen from his window, and before reading this thread, I felt in my mind the release of tension that comes with the satisfactory exercise of professional judgement. Admittedly there is rarely such clear calculation, rarely a convenient pavement to thrash out the calculations on, and it can take a couple of weeks in a park observing dog-walking, jogging, illicit drinking, littering, informal social gatherering and the use of short-cuts to extrapolate usage across a whole year, but in the end I have fould that a satisfactory defensible quantification can be arrived at without anything more than observation, judgement and chalk.

 

I have not used the QTRA system, although I hope to someday. I would contend nevertheless that is it possible to quantify risk and to exercise professlional judgements as risk aware rather than risk averse (my client demand nothing more or less than that) without buying in to a formal system. Others may, and probably do, differ from this and would welcome a rigorous systematic analysis of risks.

 

Much like an England Ireland rugby match, we all imagine and hope for a great sporting affray of blood, guts, derring-do and end-to-end stuff but so often a result is just ground out on penalties. OK, that's a bit of a dodgy analogy and another Radio 2 stylee link, but I shall leave this oblique answer to my own question at that and will look in on this thread when next I get the chance as I have found it quite illuminating so far.

Edited February 11, 2013 by daltontrees