Quantified Tree Risk Assessment (QTRA) Version 5 - Questions & Answers

[daltontrees]

Try this;

 

Take 30 unbiased coins.

 

Toss each one in turn.

 

On the table you now have 30 coins with 15 coins tails up and 15 coins heads up.

 

Is the probability of 15 of the 30 coins being tails up?

 

15/30 = 1/2?

 

Or is it

 

1/2 x 1/2 etc 15 times = 1/33 000?

 

Cheers

 

Acer ventura

 

I tried to derive a specific rather than a general proof for this and ended up looking at Fibonacci numbers and from there to Pascals Triangles. They seem to provide a general solution for any probability problem of this type. And they show that the probability of any nominated 15 coins coming up tails and the others heads will be the 1/33,000 figure but the probaility of any 15 being tails is 155,117,520/310,235,040 better known as 1/2.

[Acer ventura]

I would imagine that as the event has already happened the probability is 1.

 

Hi Tony

 

I was a 110% sure you were going to say that.

 

Good call on my wrong use of tense.

 

Cheers

 

Acer ventura

[Amelanchier]

Ah it was low hanging fruit. And a way of avoiding the question!

 

Isn't the probability of 15 out 30 coins (yet to be tossed ) ending tails up is whatever proportion of combinations that makes up out of all possible combinations?

 

So 2^30 gives us the number of possibilities and 30!/(15!*15!) gives us the number of those possibilities that contain 15 tails so (30!/(15!*15!)/(2^30) giving us P=0.14?

 

That is kind of my limit with numbers though so tread softly, because you tread on my dreams.

[Acer ventura]

A bit of tidying up from here...

 

http://arbtalk.co.uk/forum/general-chat/73829-isa-best-management-practices-tree-risk-assessment-traq-5.html#post1113787

 

...which I had forgotten about.

 

Ah but QTRA expresses relative damage or harm as a proportion of the value of a statistical life, does it not, and if so everything should be related to that standard and the term 'harm' is I would suggest (strictly speaking) inappropriate.

 

No it doesn’t. A life is a life. A risk of 1/10 000 for the loss of a life is an advisory threshold of tolerable risk. The Value of Statistical Life (VOSL) is used to apportion a monetary value of damage to property, as a fraction of the VOSL, and determine whether a risk is ALARP.

 

The Risk of Harm was coined because it covers, fatality, injury, and damage to property.

 

Cheers

 

Acer ventura

[Acer ventura]

A bit of tidying up from here...

 

http://arbtalk.co.uk/forum/general-chat/73829-isa-best-management-practices-tree-risk-assessment-traq-5.html#post1113628

 

...which I had forgotten about.

 

I have nothing further to say to QTRA about Monte Carlo simulations. They do little more meaningful than standard deviations can do, unless you assume (unintuitively, in my opinion) that risk is preceived on a linear scale.

 

Eh? This makes no sense. You need the Monte Carlo simulations to generate probability distributions before you can define the standard deviations for it.

 

The Palisade website has an easy to grasp description of what Monte Carlo simulations do;

 

Monte Carlo Simulation: What Is It and How Does It Work? - Palisade

 

I have no idea what you mean by “risk is preceived on a linear scale”.

 

Cheers

 

Acer ventura

[Acer ventura]

 

I've just had a read through v5 practice note. Thanks for bringing it to my attention, it looks much better than previous versions.

 

After reading practice note 5 I am very tempted to do a U-turn and sign up to QTRA (if you'll still have me)

 

Hi James

 

Despite Jules' tin foil hat conspiracy theories, whether QTRA will 'have you' is absolutely nothing to do with me.

 

but I'd want a bit more info on how the calculations work first. Any chance of some info on the monte carlos method used?

 

As I said somewhere else' date=' Mike Ellison hired a mathematician to sort out the Monte Carlo simulations for version 5 because it was way beyond the abilities of the loose network of contributors who work behind the scenes to help develop QTRA; myself included. What I can tell you is that there are 10 000 iterations for each range combination of Target, Size, and Probability, and that the mean from a normal distribution for each combination is the Risk of Harm.

 

If you want more information then contact Mike direct [email'][email protected][/email]

 

IIRC you had a massive aversion to VOSL on the UKTC some years back. If you don't mind me asking, what's made you change your mind about VOSL because it's still a key component.

 

Cheers

 

Acer ventura

[daltontrees]

 

Eh? This makes no sense. You need the Monte Carlo simulations to generate probability distributions before you can define the standard deviations for it.

 

The Palisade website has an easy to grasp description of what Monte Carlo simulations do;

 

Monte Carlo Simulation: What Is It and How Does It Work? - Palisade

 

I have no idea what you mean by “risk is preceived on a linear scale”.

 

Cheers

 

Acer ventura

 

A bit of tidying up of my own.

 

The Standard Deviation is the standard unit of measurement in statistics. It is rthe average amout by which scores in a distribution differ from the mean or average score. Therefore if the iterations used for the Monte Carlo simulations are evenly distributed on a logarithmic scale the standard deviation (which is the single figure that describes the shape of the resultant distributions) could have been calulated rather than generated by a simulation. And the most useful result would be the production of 'confidence levels' that would stand as evidence of the precision of QTRA. Which would be a good thing. But if you're stuck on linear scales, simulate away. Personally I don't trust it because I can't see it or its inputs.

[daltontrees]

Despite Jules' tin foil hat conspiracy theories, whether QTRA will 'have you' is absolutely nothing to do with me.

 

No theories about conspiracy, I merely said it benefits you to have QTRA succeed and its competitors fail. Anybody who cares can make up their own mind.

[Acer ventura]

Hi

 

By way of an update that might be of interest if you find yourself at the end of this thread. I've uploaded my presentation 'Safety in Numbers: Balancing Risks with Benefits', which I delivered at the International Society of Arboriculture Conference in Florida, 2015, here:

 

http://arbtalk.co.uk/forum/general-chat/88697-tree-risk-assessment-traq-qtra-compatibility-common-ground-2.html#post1347612

 

The presentation is a useful introduction to some key concepts about risk, and the value of using numbers rather than words when assessing and managing tree risk. It also covers the main developments in version 5 of QTRA.

 

Cheers

 

Acer ventura