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

[Albedo]

How about this for some maths Tony.

 

I go to the shop every night to pick up a bottle of vino tinto

 

There's about 365 days in a year so it will take me 27 years to go there 10,000 times.

 

One of those times I might trip over and hurt myself.

 

I could assign this level of risk to every tree in the land.

 

In fact somewhere during that time I'll probably get beaten up by a gang of 5 year olds too:001_smile:

 

Have I affirmed the consequent again?

[daltontrees]

Sorry I haven't been able to reply before now.

I don't know what QTRA is if it is a brand of risk assessment. I do risk assessments. I am not assessing the risk of a tree failing I am assessing the risk of someone being sertiously hurt by a tree failing. If that's what QTRA does, that is basically what I am doing.

Stupid example to illustrate the arithmetic ...

Tree with kretzschmaria bursting forth from every crevice, and I m ean proper soft-rot-brittle-failure-likely. Upwind from a well used footpath. There's a 50:50 chance of the tree harming someone (or worse) while they ar ein the danger zone. So, probability of harm 0.5

It can be obseved that on average someone passes once a minute. They will be within the death zone for 15 seconds each. So, there is a target present 1/4 of the time, or to put it another way probability of a target being present at time of failure 0.25. Probability of tree couping over prior to next scheduled inspection 1 in 10 or 0.1.

Risk 0.25 x 0.5 x 0.1 = 0.0125 or 1:80.

Way way above the specified 1:10,000.

That tree would be phoned in for removal within 48 hours. The arithmetic for that timescale is not much more complex.

It is easy to see how things are never that simple. What about during the night? Don't trees fail usually in strong winds when there are less people using paths etc. etc.?

And I am doing VTAs so there is an imperative to instruct expert examination, climbing inspection, intrusive testing etc.

I am just AA Tech and PTI, also a tree surgeon, which seems to be more than enough for the client, I think I have a little knowledge to spare beyond the requirements of the contract.

I should add that I am surveying great long stretches of footpaths, streets, school boundaries etc. where the target probability needs only to be estimated once.

All in all, Is till say that if you can't at least try to quantify the risk and have athreshold to work to you would stand the bottom of every tree, and possibly eventually at the front of a courtroom someday, umming and ahhing in a most unseemly and unproductive manner.

[RobArb]

Quantified Tree Risk Assessment: Discussion :: Public Discussion :: Accuacy of data

 

Hers's a good read:biggrin:

 

The one you where after Paul (albedo)?

 

Paul B, yup i tend to agree here with ya:thumbup:

 

Tony, out of curiosity have you any qualifications in Psychology/Philosophy or the like??

[RobArb]

Sorry I haven't been able to reply before now.

I don't know what QTRA is if it is a brand of risk assessment. I do risk assessments. I am not assessing the risk of a tree failing I am assessing the risk of someone being sertiously hurt by a tree failing. If that's what QTRA does, that is basically what I am doing.

Stupid example to illustrate the arithmetic ...

Tree with kretzschmaria bursting forth from every crevice, and I m ean proper soft-rot-brittle-failure-likely. Upwind from a well used footpath. There's a 50:50 chance of the tree harming someone (or worse) while they ar ein the danger zone. So, probability of harm 0.5

It can be obseved that on average someone passes once a minute. They will be within the death zone for 15 seconds each. So, there is a target present 1/4 of the time, or to put it another way probability of a target being present at time of failure 0.25. Probability of tree couping over prior to next scheduled inspection 1 in 10 or 0.1.

Risk 0.25 x 0.5 x 0.1 = 0.0125 or 1:80.

Way way above the specified 1:10,000.

That tree would be phoned in for removal within 48 hours. The arithmetic for that timescale is not much more complex.

It is easy to see how things are never that simple. What about during the night? Don't trees fail usually in strong winds when there are less people using paths etc. etc.?

And I am doing VTAs so there is an imperative to instruct expert examination, climbing inspection, intrusive testing etc.

I am just AA Tech and PTI, also a tree surgeon, which seems to be more than enough for the client, I think I have a little knowledge to spare beyond the requirements of the contract.

I should add that I am surveying great long stretches of footpaths, streets, school boundaries etc. where the target probability needs only to be estimated once.

All in all, Is till say that if you can't at least try to quantify the risk and have athreshold to work to you would stand the bottom of every tree, and possibly eventually at the front of a courtroom someday, umming and ahhing in a most unseemly and unproductive manner.

 

 

Don't agree with you here, trees can fall/fail more than one way, not 50% one way or 50% the other!? So looking at it that the tree WILL (50%) or WON'T (50%) hurt someone is a biased view IMO:biggrin:

 

Also how can we average people walking past a certain point at once a minute? is this over 24hrs? is this over a working day? What if a large group of runners/cyclists suddenly came past? If you start off at a bad example the argument kind of loses its depth, not that i'm trying to have a go at you:blush: just raising a point:biggrin:

[Le Sanglier]

Sorry I haven't been able to reply before now.

I don't know what QTRA is if it is a brand of risk assessment. I do risk assessments. I am not assessing the risk of a tree failing I am assessing the risk of someone being sertiously hurt by a tree failing. If that's what QTRA does, that is basically what I am doing.

Stupid example to illustrate the arithmetic ...

Tree with kretzschmaria bursting forth from every crevice, and I m ean proper soft-rot-brittle-failure-likely. Upwind from a well used footpath. There's a 50:50 chance of the tree harming someone (or worse) while they ar ein the danger zone. So, probability of harm 0.5

It can be obseved that on average someone passes once a minute. They will be within the death zone for 15 seconds each. So, there is a target present 1/4 of the time, or to put it another way probability of a target being present at time of failure 0.25. Probability of tree couping over prior to next scheduled inspection 1 in 10 or 0.1.

Risk 0.25 x 0.5 x 0.1 = 0.0125 or 1:80.

Way way above the specified 1:10,000.

That tree would be phoned in for removal within 48 hours. The arithmetic for that timescale is not much more complex.

It is easy to see how things are never that simple. What about during the night? Don't trees fail usually in strong winds when there are less people using paths etc. etc.?

And I am doing VTAs so there is an imperative to instruct expert examination, climbing inspection, intrusive testing etc.

I am just AA Tech and PTI, also a tree surgeon, which seems to be more than enough for the client, I think I have a little knowledge to spare beyond the requirements of the contract.

I should add that I am surveying great long stretches of footpaths, streets, school boundaries etc. where the target probability needs only to be estimated once.

All in all, Is till say that if you can't at least try to quantify the risk and have athreshold to work to you would stand the bottom of every tree, and possibly eventually at the front of a courtroom someday, umming and ahhing in a most unseemly and unproductive manner.

 

With respect to the other posters on here, that is the clearest and best explanation so far, and.....it did not make my head hurt, Thanks

ps I think Robarb you are perhaps being deliberately obtuse:001_smile:

Edited November 28, 2012 by Le Sanglier

[RobArb]

With respect to the other posters on here, that is the clearest and best explanation so far, and.....it did not make my head hurt, Thanks

ps I think Robarb you are perhaps being deliberately obtuse:001_smile:

 

I prefer counter-argumentative:laugh1:

[Le Sanglier]

I prefer counter-argumentative:laugh1:

 

Touché

[daltontrees]

Rob Arb, I think I pointed out the likely flaws myself, I know it's not perfect and the assessments i find myself doing on site get a lot more complicated. But I STILL say if you don't try to quantify you get nowhere.

 

I don't want to be a case law bore but the whole thing was brought into stark relief in bowen and others v national trust, kind of backs up the benefits of having a reasonably robust basis for assessing risk. I would hate for someone to be hurt by a tree on my watch, but if one were up in front of queens counsel for negligent tree assessment resulting in deaths and one presented the defence that one thought the tree was ok because one knows about this sort of thing, QC would shred you like you had been tossed into a chipper head first. Even an expert (I mean Lonsdale standard) would get a grilling.

 

I wouldn't hide behind risk assessments, the courts (see bowen) know treees are fickle and would recognise that by using a reasonably robust system a defendant had done all that was reasonable. I think tht that's more or less what the Occupiers Liability Act says.

[daltontrees]

oh and another thing... and i am being counter-counter-argumentative here... what reason might I (or anyone) have to be biased? unless i am secretly a sales agent for QTRA.

[Paul Barton]

Ouch. So how do you describe the risk with words? High, medium, low?

 

More or less, yes, although there are a few more options than that and I use a simple matrix to 'calculate' the rating so I'm happy I could defend myself in the box if needed.