Quantified Tree Risk Assessment (QTRA) - Questions & Answers

[daltontrees]

Correction, I see from UKTC that you are still in Australia, hopefully we will hear from you in due course. For my part I only have one remaining question about QTRA which I will post when I get a little time.

[Acer ventura]

Correction, I see from UKTC that you are still in Australia, hopefully we will hear from you in due course.

 

Hi Jules

 

I am back now but haven't had the time to respond. Hopefully, I'll post some replies this week.

 

Cheers

 

Acer ventura

[daltontrees]

I may as well put my last question to you.

 

As you know, and I mention it largely for the benefit of onlookers, QTRA puts each of the three ingredients (Target, Harm and Probability of Failure) into ranges of probability for example 10 to 36 pedestrians an hour is taken as 1/20 target presence, which is the upper end (36 pedestrians an hour). To put it another way, it errs on the cautious side.

 

This resultant probability is then multiplied by the other factors, which also are rounded in the 'cautious' direction . It appears to me that this has the potential to result in super-cautious conclusions from the QTRA calculator. I took some figures at the extreme lower end of each of the three factors and multiplied them together. I compared this with the product of 3 figures from the upper end of the same range. It is not difficult to find examples where the difference between them is a factor of 2,000. The difference between them in one scenario was a factor of 3,000.

 

Since all QTRAs figures are rounded on the cautious side, what this seems to me to result in is the potential for QTRA to recommend remedial tree works for trees where the arithmetic for the actual constituent parts would not result in such a recommendation.

 

So, for example, a tree is calculated by QTRA as presenting a 1:7,500 risk, which is considered by HSE therefore to be an unacceptable risk; let's say the remedy would be felling. However, the actual figures could when multiplied together be say 2,500 times less, equalling 7,500 x 2,500 = 1:18,750,000, putting the tree in the 'broadly acceptable' category. No work to the tree would be required.

 

These two completely different results for the same tree appear to support the view some have that QTRA produces false precision which (with these rounding discrepancies) can lead to false accuracy.

 

There is a question... how does the practitioner or the client or someone like myself who is a potential future customer of QTRA reconcile this all so that acceptable trees do not get felled?

[Acer ventura]

Hi

 

I'm about to post a bundle of replies to the open questions. I’ve tried to break the issues down into easy to digest posts, so that they’re easier to follow and reply to.

 

Regarding moving targets…I try to take account of increased target presence time for wider canopies. The difference between a Lombardy Poplar and a Horse Chestnut of the same DBH for example could be considerable.

 

As regards height of fall, again I would tend to differentiate when if the shape of the canopy allows me to. A clear fall of a high-up snow-laden Scots Pine limb (close-grown examples of said species can commonly be 20 m high with canopy restricted to last 5m) onto a road I think deserves higher rating of 'IP' than same size limb lower down.

 

…I always take a cautious view when surveying near primary schools where children could not withstand the injury adults could.

 

Personally I feel able to distinguish between tree risk of death and lesser risks, from being flattened outright by a tree trunk right down to getting a poke in the eye from a protruding branch.

 

The worst outcome barring freak occurences and particularly fragile targets would be laceration and mental scarring. If the tree failure had been foreseeable and nothing had been done about it the landowner would be negligible and responsiple for recompense to the victim.

 

If the tree failure had been foreseeable

 

Hi Jules

 

I don’t wish to sound impolite, but earlier in this thread you were very adamant that you didn’t want to discuss your take on quantifying tree risk. I have some serious reservations about some of the things you are saying, or claiming to be able to do, in your last few posts, as you gradually reveal some of the stitches and patterns of your home-knitted approach to quantifying tree risk. I’ve highlighted a couple of points of concern above. Can we agree that you don’t say what your take on it is if you don’t want me to comment on it? Unless you’ve changed your mind and do want to discuss your version?

 

Cheers

 

Acer ventura

[Acer ventura]

I am taking as read the 1:1,000,000 and 1:10,000 brackets that HSE has suggested as guidelines. To make it really really clear, this is the guidelines for risk of death. This seems to be ingrained in QTRA calculations.

 

As well as the HSE, The National Tree Safety Group (NTSG), the British Medical Association, The Royal Society, NASA, and all other space agencies (though Iran and North Korea are probably doing their own thing) all propose the 1/10 000 or 1/1 000 000 levels of risk as being ‘tolerable’ or ‘acceptable’.

 

In QTRA language, the ‘H’ in RoH is ‘Harm’ not D ‘Death’

 

To quote QTRA Guidance;

 

“QTRA Risk of Harm 1/10 000

 

An advisory threshold of ‘tolerable’ risk where it is imposed on the public in the wider interest; providing the risk is ALARP.

 

QTRA Risk of Harm 1/1 000 000

 

An advisory threshold of ‘acceptable’ risk. No need to consider whether the risk is ALARP."

 

1/10 000 is not ‘ingrained’ in QTRA calculations. QTRA measures the components of tree risk as probabilities in order to express the ‘Risk of Harm’ (RoH) as a probability so that the risk manager/owner can consider it against these published thresholds. They don’t have to manage their tree risk to 1/10 000 or 1/1 000 000, they could choose any level of risk they wish. All we are suggesting is a reasoned and reasonable basis to adopt these thresholds for the tree manager/owner, and that the risk assessor propose them in their reports.

 

I have worked on large tree surveys where the instruction has been to satisfy the client's Duty of Care responsibility. Had I used the unacceptable/tolerable/broadly acceptable verdicts of the QTRA system or indeed a home-knitted version of it' date=' I could have been far far off the mark and have failed the client.[/quote']

 

You’ve lost me here. Are you saying 1/1 000 000 RoH is not an acceptable risk and 1/1 00 000 RoH is not a tolerable risk as long as it’s ALARP? And that to manage risk to these levels is in breach of their duty of care. Can you expand please?

 

Cheers

 

Acer ventura

[Acer ventura]

However, does not the common law and the Occupiers Liability legislation create an obligation to avoid foreseeable risk of injury or damage?

 

I don’t think it does.

 

Common law duty of care

 

“…to take reasonable care to avoid acts or omissions which you can reasonably foresee would be likely to injure your neighbour”

 

Occupiers’ Liability Act (1957 & 1984)

 

“…a duty to take such care as in all the circumstances of the case is reasonable to see that the visitor will be reasonably safe in using the premises for the purposes for which he is invited or permitted by the occupier to be there.”

 

The words ‘avoid’ and ‘foresee’ are importantly qualified by a range of ‘reasonables’, and an all-important ‘likely’, and they aren’t in the Occupiers’ Liability Act at all; though notably there’s another couple of ‘reasonables’ there.

 

In relation to the UKTC debate about this:

 

Foresee = Prediction

 

Reasonably foresee would be likely = Probability

 

Beyond that, and in particular relating to the Health & Safety at Work Act (1974), and various risk assessment and risk management guidance, we also need to embrace ‘reasonable practicability’ and ‘proportionality’.

 

Cheers

 

Acer ventura

[daltontrees]

Hi

 

I don’t wish to sound impolite, but earlier in this thread you were very adamant that you didn’t want to discuss your take on quantifying tree risk. I have some serious reservations about some of the things you are saying, or claiming to be able to do, in your last few posts, as you gradually reveal some of the stitches and patterns of your home-knitted approach to quantifying tree risk. I’ve highlighted a couple of points of concern above. Can we agree that you don’t say what your take on it is if you don’t want me to comment on it? Unless you’ve changed your mind and do want to discuss your version?

 

Cheers

 

Acer ventura

 

I don't recall exactly what I said but I couldn't discuss specific cases where client confidentiality prevented it. I offered to discuss a tree where no confidentiuality exists. However, since then I expect readers will be more interested in learning about QTRA than about my methods, and if I have alluded to my methods it will be anecdotally to emphasise my point or to illustrate my question to a wider readership. So to answer your questions, you don't need to comment on my take on it, your respones to the questions are the important thing.

[Acer ventura]

As you know, and I mention it largely for the benefit of onlookers, QTRA puts each of the three ingredients (Target, Harm and Probability of Failure) into ranges of probability for example 10 to 36 pedestrians an hour is taken as 1/20 target presence, which is the upper end (36 pedestrians an hour). To put it another way, it errs on the cautious side.

 

Since all QTRAs figures are rounded on the cautious side, what this seems to me to result in is the potential for QTRA to recommend remedial tree works for trees where the arithmetic for the actual constituent parts would not result in such a recommendation.

 

Hi Jules

 

For Harm, you mean Impact Potential (Size of Part).

 

For reasons of being ‘proportional’ and ‘reasonably practicable’ within the risk assessment, with the manual calculator and the broad ranges in the software calculator, it is the highest values within the broad ranges which are multiplied. If the actual input figures are some way from those highest values in the range, then the RoH will be an overestimate of the risk. It will err on the side of safety. In most cases this does not matter because the risk is so low. As such, if refining the inputs would result in the RoH being even lower why waste time and effort coming out of the manual calculator to refine the risk?

 

Where the resulting RoH exceeds or approaches a risk threshold – say 1/10 000 – which might otherwise prompt some tree work to control the risk, then the risk assessor is advised to look closely at the inputs because of the very reason that we’re taking the highest value from the broad ranges. It might be that refining the RoH by, for example, inputting the actual Target Value, if you have confidence in its origin, results in a RoH substantially lower than 1/10 000. This can occur with high Target Values, and there’s an example of Target Range 1 (1/1 - 1/20) refinement to illustrate this point in the QTRA training day.

 

Have a look at ‘Accuracy of Outputs’ section on p. 7 of the QTRA Practice Note

 

Quantified Tree Risk Assessment

 

So' date=' for example, a tree is calculated by QTRA as presenting a 1:7,500 risk, which is considered by HSE therefore to be an unacceptable risk; let's say the remedy would be felling. However, the actual figures could when multiplied together be say 2,500 times less, equalling 7,500 x 2,500 = 1:18,750,000, putting the tree in the 'broadly acceptable' category. No work to the tree would be required.[/quote']

 

I appreciate that these figures are plucked by you to illustrate a point you’re looking to make, but just to clarify, you can’t get these RoH using QTRA. Not least because RoH is expressed to 1 significant figure; as has already been covered earlier this thread and the QTRA Practice Note.

 

These two completely different results for the same tree appear to support the view some have that QTRA produces false precision which (with these rounding discrepancies) can lead to false accuracy.

 

I think this point has been already been addressed earlier here.

 

http://arbtalk.co.uk/forum/general-chat/53668-quantified-tree-risk-assessment-questions-answers-4.html#post825414

 

...the Goode post at Post 1, and throughout the thread.

 

As an aside, on this issue of highest values within a range, one of the many things I found interesting when analysing other tree risk assessment systems, is that they all use bands or ranges, but don’t disclose whether it’s the highest value of that range, an average (and if so what average), or the lowest value, that is determining what the eventual risk outcome is.

 

One of the many reasons that QTRA chimed with me when I first encountered it was that all the components, how they are put together, and why they are put together in the manner in which they are is entirely transparent.

 

Cheers

 

Acer ventura

[daltontrees]

I don’t think it does.

 

Common law duty of care

 

“…to take reasonable care to avoid acts or omissions which you can reasonably foresee would be likely to injure your neighbour”

 

Occupiers’ Liability Act (1957 & 1984)

 

“…a duty to take such care as in all the circumstances of the case is reasonable to see that the visitor will be reasonably safe in using the premises for the purposes for which he is invited or permitted by the occupier to be there.”

 

The words ‘avoid’ and ‘foresee’ are importantly qualified by a range of ‘reasonables’, and an all-important ‘likely’, and they aren’t in the Occupiers’ Liability Act at all; though notably there’s another couple of ‘reasonables’ there.

 

In relation to the UKTC debate about this:

 

Foresee = Prediction

 

Reasonably foresee would be likely = Probability

 

Beyond that, and in particular relating to the Health & Safety at Work Act (1974), and various risk assessment and risk management guidance, we also need to embrace ‘reasonable practicability’ and ‘proportionality’.

 

Cheers

 

Acer ventura

 

As originally stated, I didn't want a debate ont eh exact wording, I merely wanted to differentiate between the thresholds of death and injury, which I expect are different for the same tree.

 

The UKTC debate on probability etc. I felt was inconclusive, but I don't want to open it here again because I (and I anticipate others) take the general point that QTRA assigns some sort of figure to likelihood/probability/foreseeability of failure and by combin ing it with other factors a figure is arrived at to represent likelihood/probability/foreseeability of harm.

 

So to clarify, my question had been about the thresholds.

[daltontrees]

 

You’ve lost me here. Are you saying 1/1 000 000 RoH is not an acceptable risk and 1/1 00 000 RoH is not a tolerable risk as long as it’s ALARP? And that to manage risk to these levels is in breach of their duty of care. Can you expand please?

 

Cheers

 

Acer ventura

 

The broad principle as we now know is that HSE says that less than 1,1,000,000 is broadly acceptable, more than 1,10,000 is unacceptable and in between is 'tolerable' and should be reduced where it is practical to do so. My question was rather about what QTRA does when the quantified overall risk of death gives the all-clear for a tree but the risk of lesser harm is possibly still unacceptable.