Quantified Tree Risk Assessment (QTRA) - Questions & Answers

[Acer ventura]

As originally stated, I didn't want a debate ont eh exact wording

 

Hi Jules

 

I appreciate that but you raised it and the exact wording is really, really important. There's a huge difference between the meaning of 'foreseeable' and 'reasonably forsee would be likely'. It's one of the foundations on which QTRA is built, which is what we're talking about on this thread.

 

Cheers

 

Acer ventura

[daltontrees]

Hi Jules

 

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.

 

Acer ventura

 

That is kind of what I expected you might say and answers the question. In effect QTRA allows the non-borderline trees to be eliminated easily leaving only the borderline ones to receive more time and detailed appraisal. That is a good thing as long as the surveyor knows to refine the iffy ones.

[daltontrees]

 

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.

 

 

Acer ventura

 

I thought calcualtor can produce numbers like that, which QTRA then rounds them to 1 significant figure. Anyway it doesn't matter, this would only exacerbate the effect I was trying to illustrate but you have clarified that a precautionary approach for outcomes signalling the potential need for action. And presumably when investigating borderline cases QTRA woudl ahve to calculate exact figures adn ignore the final rounding if the detailed outcome is still borderline?

[daltontrees]

Hi Jules

 

I appreciate that but you raised it and the exact wording is really, really important. There's a huge difference between the meaning of 'foreseeable' and 'reasonably forsee would be likely'. It's one of the foundations on which QTRA is built, which is what we're talking about on this thread.

 

Cheers

 

Acer ventura

 

Fair enough, you will want to mak your point to other readers. I get the point.

 

With one caveat, I am curious as to the source of your statement of common law. I always found Leakey v National Trust nice and clear viz. "a duty to do that which is reasonable in all the circumstances, and no more than what, if anything is reasonable, to prevent or minimize the known risk of damage or injury to one’s neighbour or to his property".

[daltontrees]

Apologies. I said I didn't want to get into the wording of the law but I also see now that you quoted the Occupiers Liability Act 1957 but there is a different duty in the Scottish Act which is very similarly worded to the 1984 Act duty towards uninvited persons. For the benefit of other readers I shall do my best to quote it here

"such care as in all the circumstances of the case is reasonable to see that the person will not suffer injury or damage by reason of any such danger [which is due to the state of the premises]"

[Acer ventura]

I thought calcualtor can produce numbers like that, which QTRA then rounds them to 1 significant figure. And presumably when investigating borderline cases QTRA woudl ahve to calculate exact figures adn ignore the final rounding if the detailed outcome is still borderline?

 

Hi Jules

 

No matter how accurate the inputs are, and as demonstrated earlier in the thread we can get very accurate traffic figures, the output RoH is only ever expressed to 1 significant figure. It’s not physically possible to get more than a 1 significant figure RoH using the manual calculator or the software calculator, no matter what level significant figure values actually go into the calculation.

 

If you’re approaching a RoH threshold, with input values that are not close to the highest value of the broad ranges, and want to refine the risk by multiplying the 3 components with a conventional calculator then you round the end RoH to 1 significant figure yourself. If you’ve cobbled together an excel spreadsheet, then you set it to round to 1 significant figure.

 

The rounding to 1 significant is never ‘ignored’. You may recollect from earlier, we always round the RoH to 1 significant figure because no matter the accuracy or levels of significant figures of the Target or Impact Potential (Size of Part), the PoF component is expressed within a range and to 1 significant figure.

 

Cheers

 

Acer ventura

[Acer ventura]

I am curious as to the source of your statement of common law

 

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”

 

…is Lord Atkin from Donoghue v Stevenson, which according to Charles Mynor’s ‘The Law of Trees, Forests and Hedgerows, “…is still the leading case on the area of civil liability.”

 

Apologies. I said I didn't want to get into the wording of the law but I also see now that you quoted the Occupiers Liability Act 1957 but there is a different duty in the Scottish Act which is very similarly worded to the 1984 Act duty towards uninvited persons. For the benefit of other readers I shall do my best to quote it here

"such care as in all the circumstances of the case is reasonable to see that the person will not suffer injury or damage by reason of any such danger [which is due to the state of the premises]"

 

The ‘duty’ in the Scottish 1960 Act doesn’t look ‘different’ to the 1957 or 1984 Act to my untrained legal eyes. We have that word ‘reasonable’ again in relation to the level of ‘care’ expected from the duty holder.

 

It seems to me that a duty holder in Scotland' date=' who manages the risk from their trees to 1/10 000 because of all the benefits they convey, so long as the risk is ALARP, would be exercising [b']reasonable care[/b].

 

Cheers

 

Acer ventura

[daltontrees]

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”

 

…is Lord Atkin from Donoghue v Stevenson, which according to Charles Mynor’s ‘The Law of Trees, Forests and Hedgerows, “…is still the leading case on the area of civil liability.”

 

 

 

The ‘duty’ in the Scottish 1960 Act doesn’t look ‘different’ to the 1957 or 1984 Act to my untrained legal eyes. We have that word ‘reasonable’ again in relation to the level of ‘care’ expected from the duty holder.

 

It seems to me that a duty holder in Scotland, who manages the risk from their trees to 1/10 000 because of all the benefits they convey, so long as the risk is ALARP, would be exercising reasonable care.

 

Cheers

 

Acer ventura

 

Donoghue v Stevenson (the snail in the ginger beer) case I remember as being ground-breaking, but Leakey seems a bit more rigorous because it relates (i) to heritable property and (ii) covers damage AND injury and (iii) allows for shades of grey in duty of care rather than black and white and (iv) clarifies that things originating on land but causing injury or damage on or to neighbouring land are included in the duty of care. I think it illustrates QTRA and ALARP principles better than Donoghue for those reasons, but that is just a personal view.

 

Regarding Occupiers Liability Acts, the scottish Act 1960 is clearest of all but I think the english 57 and 84 Acts need a bit of thought before you can quite so readily conclude that together they are aligned to the wider common law (which of course they partly replaced). The differences are small and don't seem substantive, quirks of history rather than variations of intention. So, all is well there I hope. I for one need no further discussion on it.

[daltontrees]

Hi Jules

 

No matter how accurate the inputs are, and as demonstrated earlier in the thread we can get very accurate traffic figures, the output RoH is only ever expressed to 1 significant figure. It’s not physically possible to get more than a 1 significant figure RoH using the manual calculator or the software calculator, no matter what level significant figure values actually go into the calculation.

 

If you’re approaching a RoH threshold, with input values that are not close to the highest value of the broad ranges, and want to refine the risk by multiplying the 3 components with a conventional calculator then you round the end RoH to 1 significant figure yourself. If you’ve cobbled together an excel spreadsheet, then you set it to round to 1 significant figure.

 

The rounding to 1 significant is never ‘ignored’. You may recollect from earlier, we always round the RoH to 1 significant figure because no matter the accuracy or levels of significant figures of the Target or Impact Potential (Size of Part), the PoF component is expressed within a range and to 1 significant figure.

 

Cheers

 

Acer ventura

 

I just meant that if you calculate the actual RoH as 1:975,000 and rounded this to 1 significant figure you would get 1:1,000,000 and no action is required, but if you used the actual figure you are above the 1:1,000,000 threshold and action may be required. The consequences are probably trivial, I am just exploring the perception of some people that QTRA can have false precision and therefore whether this can result in false accuracy. The word 'ignored' was used by me in the mathematical sense, i.e. the treatment as insignificant of the smaller order digits in a rounded number.

I am happy to leave the debate about precision there.

[Acer ventura]

I just meant that if you calculate the actual RoH as 1:975,000 and rounded this to 1 significant figure you would get 1:1,000,000 and no action is required, but if you used the actual figure you are above the 1:1,000,000 threshold and action may be required.

 

Hi Jules

 

If you’re using QTRA, the ‘actual RoH’ here is 1/1 000 000. 1/975 000 is not a RoH. It’s an incomplete calculation of the RoH, for the reasons I've explained previously.

 

If you didn’t want to use QTRA to assess tree risk, and put together another method to measure the risk to such a precise probability of 3 significant figures and got 1/975 000. Then, with a risk at 1/975 000, and taking the Value Of Statistical Life at £1 000 000, action would only be required if it cost less than £1.03 (rounded to pennies) to bring the risk down to 1/1 000 000. If you have to spend more than £1.03 the risk is ALARP and no action is required. If you go ahead and spend more than £1.03 you’re being ‘grossly disproportionate’.

 

The consequences are probably trivial

 

Indeed. With QTRA a RoH of 1/900 000 is ALARP unless the cost to reduce the risk to 1/1 000 000 is less than £1.11.

 

Cheers

 

Acer ventura