QTRA - I'm sorry i don't agree with it!

[daltontrees]

sorry, finaL comment before I put the wean to bed. I should ahve read the counter argument properly.

I didn't say bad example, I said stupid. Does it put my posting in a different context if I say a 'grossly oversimplified example selected for its illustrative purposes'?

When I am looking at trees I trry and look atthe whole tre. The canopy for vigour, every branch, limb, fork, every bit of the stem and as much of the roots and butresses as possible, and I stamp about a bit to see if there is anything telling under the ground. What my purpose in that is (others may take a different approach) I am looking at every possible way the tree or any bit of it can fail. And then how likely that failure is. Then I pick the most likely failure. That is a start for quantifying one part of the risk i.e likelihood of failure. And the size of treh part that can fail and the height it will fall from points to the seriousness and likelihood of harm. Then I stand there for 3 months to see how many people go past.

[oslac]

If a hazard assessment is being carried out for a local authority, I would expect a strategy to be agreed before-hand as to which trees require surveying and when based (probably) on the target area for each tree (zoning). This becomes objective in the field but the strategy should be carried out by an experienced and qualified arby and as such is based on that arby's experience and understanding.

 

Deciding on the size of the potential failure (tree or branch) is reasonably simple and can be objective but obviously a 5cm diameter branch falling from two metres won't do half as much damage to your head as the same sized branch falling from 20 metres. So failure size is objective but potential to cause injury or damage is subjective.

 

The risk of failure is, in my opinion, subjective and based on expertise and knowledge. The arby can be given pointers as to what to look out for but interpreting this knowledge correctly usually takes a lot of experience and and a lot of knowledge.

 

So what I am trying to say is that anyone carrying out tree risk assessments should be very qualified and very experienced and have a system in place which is applied to all trees within the survey. If this system is QTRA then ok but there are other systems which are just as good and do not require buying a licence.

 

All decision making is subjective and tree risk assessments are very subjective . . . don't hide behind QTRA at the expense of building up your knowledge and experience and if you have knowledge and experience, you probably won't need QTRA.

 

Just a thought:001_smile:

[daltontrees]

Ahh, oslac, just when I was going to say I concur with you and tha tthat was a comfortable juncture for patting myself on the back for not wasting a license fee for QTRA, I realised there is a serious message in this somewhere.

I hate to drag in the National Tree Safety Group stuff, but NTSG does rather fancy itself as having arriveds at the definitive guidance on the matter and goes as far as to say it hopes its findings are adopted in the courts. Anyway the distinction needs to be made between the landowner, the inspector and the assessor of risk. NTSG suggests the landnowner should identify which trees, which if the fell, would harm someone. An inspection by a bod who need only have a working knowledge of trees (not necessarily and arboriculturalist) should follow and if defects are noted the risk should be quantified by an arboriculturalist and then mitigating action taken where appropriate.

3 separate acts by 3 different classes of people there. Agreed the assessor of risk should have 'appropriate knowledge and expeerience' (to quote NTSG).

Teh contract I am working on requires someone like me to do all 3. With the fallback of referral of unusual cases to experts, a call for a climbing inspection or tomography etc.

Hope that makes things a little clearer.

[oslac]

The NTSG website links to this document

 

http://www.forestry.gov.uk/PDF/FCMS025.pdf/$FILE/FCMS025.pdf

 

Common Sense Risk Management of Trees . . . No mention of QTRA there . . .

 

Its all well and good using a bod to identify tree hazards, I guess they will be armed with a series of photographs showing potential defects and if one or more of these defects are found, reporting this to a more qualified arby who will carry out a further inspection. Lets hope the bod doesn't miss something whose target is over something precious like my son on his way to school, or me coming home from the pub.

 

Carrying out many tree inspections per year would probably arm you with a certain instinct about trees and their potential for failure. I just don't think that QTRA is anything more than another expensive scheme to assess risk. I expect someone is very proud of their achievement and has been rewarded handsomly in financial terms and a massive ego boost . . . I wish I had thought of it first . . .

Edited November 28, 2012 by oslac

[daltontrees]

That's the abridged version of the full-size NTSG doc, which is a dull dull 90 pages or so.

Wisely said to me once that to know what a dodgy tree looks like you need to know what a normal one looks like. Doing lots of tree surveys gives you a chance to do that. But the most useful thing I find is to spend time looking at recent failures (before the deadwood fungi get stuck) in and to get to know for example how much included bark causes failure of compression forks, what extent of fungal hollowing precedes stem failure, how the amount of k. deusta at the surface relates to the extent of decay inside, what abnormalities of bark correspond with what kinds of failures. When you see it again on a standing tree the right alarm bells ring.

I have had a look at the QTRA website now, it looks like a system that might have been cutting edge (in terms of risk assessment) once but I can't see many people taking it up for money now. If nothing else NTSG seems to have demystified the whole thing. In a dull dull way.

[RobArb]

The NTSG website links to this document

 

http://www.forestry.gov.uk/PDF/FCMS025.pdf//FCMS025.pdf

 

Common Sense Risk Management of Trees . . . No mention of QTRA there . . .

 

Its all well and good using a bod to identify tree hazards, I guess they will be armed with a series of photographs showing potential defects and if one or more of these defects are found, reporting this to a more qualified arby who will carry out a further inspection. Lets hope the bod doesn't miss something whose target is over something precious like my son on his way to school, or me coming home from the pub.

 

Carrying out many tree inspections per year would probably arm you with a certain instinct about trees and their potential for failure. I just don't think that QTRA is anything more than another expensive scheme to assess risk. I expect someone is very proud of their achievement and has been rewarded handsomly in financial terms and a massive ego boost . . . I wish I had thought of it first . . .

 

 

(apart from the thinking about it first bit.... had i thought of it first i would of past it off as whimsical:biggrin:)

[Amelanchier]

Some decent sensible points there - I'm glad we've got away from the idea that QTRA is purposefully deceptive.

[HCR]

sorry, finaL comment before I put the wean to bed. I should ahve read the counter argument properly.

I didn't say bad example, I said stupid. Does it put my posting in a different context if I say a 'grossly oversimplified example selected for its illustrative purposes'?

When I am looking at trees I trry and look atthe whole tre. The canopy for vigour, every branch, limb, fork, every bit of the stem and as much of the roots and butresses as possible, and I stamp about a bit to see if there is anything telling under the ground. What my purpose in that is (others may take a different approach) I am looking at every possible way the tree or any bit of it can fail. And then how likely that failure is. Then I pick the most likely failure. That is a start for quantifying one part of the risk i.e likelihood of failure. And the size of treh part that can fail and the height it will fall from points to the seriousness and likelihood of harm. Then I stand there for 3 months to see how many people go past.

 

Vitality, surely

 

On a more serious note, good series of posts. And what you describe is all that is required.

 

I can see nothing wrong with QTRA, but I don't see that it is needed either. There isn't even a need to directly quantify the risk (whether it's high/medium/low, traffic lights or whatever). Specifying further action will keep the courts happy - re-inspect in 3 years; remove within 48 hours; fence off danger area and divert footpath etc etc.

 

Then if the worst comes to the worst, you have to satisfy a court that on the balance of probability (or in extremis introduce a reasonable doubt) that a reasonably competent tree inspector wouldn't have recommended more precautionary work than you would have.

[Albedo]

Some decent sensible points there - I'm glad we've got away from the idea that QTRA is purposefully deceptive.

 

My point has been that you can’t be subjective and objective at the same time.

 

Some of the assessment can be objectively accurate, like land use and frequency of targets, and some of it has an element of subjective judgement.

 

So you can assign a fairly accurate number to parts of the assessment but not to others.

 

The problem would be in presenting one objective number or probability for an assessment based on data which is in part subjective.

 

I am quite sure that intention to deceive has not been mentioned.

 

More that the drive to fill the brief of assessing risk may have resulted in the development of a system, which by presenting risk as an overall numerical probability (If this is indeed the case) , gives the impression of a degree of objectivity that is not consistent if you follow the data trail to its source. (not consistent because subjective and objective observations are mixed)

 

 

If this is not the case then I will stand corrected. That’s why Tony used my doctor analogy, in that it is acceptable to assign a number on a scale of 1-10 to a subjective feeling of pain in a patient.

 

So therefore it’s acceptable to place a numerical probability on risk in a situation where people are exposed to failure from trees using, in part, subjective data.

 

Affirming the consequent, sometimes called converse error, is a formal fallacy, committed by reasoning in the form:

1. If P, then Q.

2. Q.

3. Therefore, P.

An argument of this form is invalid, i.e., the conclusion can be false even when statements 1 and 2 are true. Since P was never asserted as the only sufficient condition for Q, other factors could account for Q (while P was false).

The name affirming the consequent derives from the premise Q, which affirms the "then" clause of the conditional premise.

Edited November 29, 2012 by Albedo
added bold

[daltontrees]

I get it Albedo, I think. P Trees fall over. Q It's a tree, so it is going to fall over.

 

Yes HCR, I should have said vitality. Old habits fie hard, I always have a look to see whether twig extension this year is more or less than last year or less than noirmal for species. But I have never concluded anything from it that couldn't be worked out from unseasonal weather or some obvious defect like Armillaria. Thanks for the correction though, accuracy beats precision every the time.