We know from the available data that the overall risk from Summer Branch Drop is mindbogglingly low.
The risk is so low, our risk of death from driving for the few minutes it takes to cover 5km/3mi is higher than from Summer Branch Drop over a whole year.
Yet, fear-mongering and threats from risk entrepreneurs about legal action if such a mindbogglingly low risk happens are pushing some anxious duty holders into wasting time and money on putting up confusing and ineffective signs; like this recent photo at Scone Palace, Scotland.
Not only are the costs of commissioning and managing these signs grossly disproportionate to any questionable reduction in a risk that's already Acceptable. Their ineffectiveness may backfire on the duty holder and create an opportunity for a risk entrepreneur to act as an expert in the extremely unlikely event of someone being injured or killed. It'd be all too easy for them to claim the duty holder hadn't managed the risk well enough with signs and they could've reduced it more by fencing or pruning. Or if the visitor wasn't an English speaker, why wasn't the sign translated into their language.
If you're a duty holder, or advise one, worry not about Summer Branch Drop and where, when, and how many signs to put up - or how many languages to use. We've got you covered with our handy Summer Branch Drop Guide on our Risk Management page.
It’s always valuable to see how others view your work because they’re seeing it with different eyes.
We were recently asked by a State Government client to publish a standalone summary of our Tree Risk-Benefit Management Strategy because it was the easiest way for their senior managers to understand how they were updating their approach to managing the risk.
The Strategy Summary is now available on the home page of our website.
We're updating all our publications from Tree Risk-Benefit Assessment & Management to Tree Risk-Benefit Management and Assessment.
We thought we'd gotten past looking at tree risk through the eyes of an Arborist, rather than a Duty Holder, with v5.0 of the Strategy. This is where creating a flowchart helped reveal that the most valuable risk management asset is Passive Assessment by anyone. Not Active Assessment by an Arborist.
A 5 yearly Active Assessment isn't being topped up by Passive Assessment, as we used to propose. Passive Assessment is being carried out all of the time, day in day out. It's being topped up by Active Assessment in zones of high confluence every 5 years.
Well, in the process of completing our work helping the Tasmanian Government manage their tree risk, David was asked whether we could produce a summary to explain what VALID is about. It's for the strategic decision-makers, they explained.
"'What is VALID?', does that", David said.
"In 'What is VALID?', the first picture is of a woman looking at a VALID likelihood of failure decision in the Tree Risk App. That's an Active Assessment at a Detailed level", they replied.
"That level of investigation is not the first thing in the minds of Tasmanian Government decision-makers when it comes to adopting VALID's approach to Tree Risk-Benefit Management, is it?"
Hence the upgrade and the new, 'What is VALID?'
Jeremy Barrell’s Arb Magazine version of his journal article has been posted by the Arboricultural Association, on their website, as a 'Legal Update' - it's not a legal update. It's had some Duty Holders and Arborists ask us about where his binary, ‘High’ or ‘Low’ likelihood of occupancy approach sits with VALID's likelihood of occupancy categories.
To recap. The heart of the article is three legal cases cherry-picked by Jeremy Barrell, where Jeremy acted as an expert, and are Jeremy’s interpretations of these cases where Jeremy’s evidence was a key factor.
When we first went through the article, we were disappointed to find it appeared to be less about reasonable, proportionate, and reasonably practicable tree risk-benefit management (which is surely what the Courts and Coroners are after). And more about setting out what Jeremy expects to contest when he's employed as an ‘expert witness’.
Back to the binary ‘High’ v ‘Low’ likelihood of occupancy.
VALID’s likelihood of occupancy categories are based on log base 10, like the Richter scale uses to measure earthquakes. If we show the likelihood of occupancy to scale as 10 x 10 canvases, and set High at the centre, we can compare VALID’s and Jeremy’s likelihood of occupancy categories.
Jeremy’s High occupancy spans 4 Richter Scale orders of magnitude with a range of ×10 000!
Or, another way of looking at it. A Richter scale 4 earthquake is categorised the same as a Richter scale 7 earthquake.
We know from pedestrian data the centre of built-up areas have a likelihood of occupancy that means on average more than one person is exposed to the risk. Occupancy is Very High.
We also know the busiest roads have a likelihood of occupancy that means on average more than one vehicle is exposed to the risk. Occupancy is Very High.
Where we have busy roads next to busy footpaths in towns and cities, we know that the combined occupancy of people and traffic means on average more than one person AND one vehicle is exposed to the risk. Occupancy is Very High.
From the training we’ve delivered to those who have upgraded their approach to tree risk, we know that tree risk assessors have been poorly trained to recognise Very High and High Occupancy.
What all this means is that unless you’re using VALID, and have had likelihood of occupancy training (it’s really easy), you’ll be undervaluing the risk where it matters most.
You’ll most likely be undervaluing the risk by at least a whopping factor of x10 just on the ‘targets’ you’ve chosen.
In the previous post we looked at VALID's Goldilocks Likelihood of Occupancy canvas to explore categories that are not too wide, and not too narrow, that are just right.
This Likelihood of Occupancy canvas is useful to examine tree risk management and assessment decision-making at the High Court in the UK’s landmark Poll v Bartholomew Judgment.
In Poll, a motorcyclist was seriously injured by a falling Ash stem. The Judge found for the Claimant because the tree was ‘High Risk.’
In their reports, the Claimant’s expert said the tree was ‘High Risk’ and the Defendant’s expert didn’t mention any level risk. Yet in their Joint Statement, the experts agreed the tree was a ‘Medium Risk.’
Naturally, the expert’s opinions left the court scratching its head and it had to ask them to produce a Second Joint Statement to define what they meant by High, Medium, and Low risk.
In the Second Joint Statement, the experts told the court the tree was ‘High Risk’. However, they concluded the risk was high after they'd assessed the Likelihood of Occupancy for a minor country road at 50%, when in fact it was 1%. The experts overvalued the occupancy by a whopping factor of 50. This gaffe was so enormous the tree was in fact a ‘Medium Risk’ and not a ‘High Risk.’
The Judge would’ve found for the Defendant if the tree was a ‘Medium Risk.’
In the Scale of the Problem we saw the overall risk from branches and trees falling is so extremely low we need a microscope to see it.
The only sensible way to measure risks that go this low is to use a logarithmic scale. It turns out the Goldilocks logarithmic scale for tree risk that’s not too narrow, and not too wide, that's just right is log base 10. Just like the Richter Scale is for measuring earthquakes.
So far so numberwang. What does that mean for you?
Well, when it comes to Likelihood of Occupancy decision-making, the advantages of log base 10 are obvious when drawn to scale. There’s 5 colour-coded Likelihood of Occupancy categories in VALID. If we centre High, then you can only see a bit of the heel of Very High. Nearly all of it’s off the screen. You can make sense of Moderate but you can barely make out Low. And you can’t see Very Low at all.
So, the first decision a Validator makes with Likelihood of Occupancy is what 3 categories can’t it be? Which 3 make no sense? Once calibrated this is an effortless decision. Then it comes down to one of two. Usually, which one is the most obvious because they're huge canvases. If in doubt you go for the higher one.
We regularly use visuals to convey both the context of tree risk and how you can measure it. Recently, we shared a post with Professor David Ball's quote that the prospects of reducing the risk below the current level were comparable to finding a microscopic needle in a gargantuan haystack.
To help convey that finding a microscopic needle in a gargantuan haystack simile further, we've had a play around with illustrating the tree risk context using coloured spectrums.
In the graphic, the top spectrum shows the reality of tree risk to scale. We know that compared to other everyday risks we readily accept, the overall risk to us from branches or trees falling is extremely low. Our annual risk of being killed or seriously injured is less than one in a million. At this scale, we can't see the amber and red risks. We'd need a microscopic.
If we take the risk spectrum to a scale where we can just make out the risks that we're trying to find and manage, we have to overvalue the base-rate risk by a factor of more than 1000.
Taking the 'defect' out of tree risk-benefit assessment
Has been published in the spring edition of the Arboricultural Association's Arb Magazine. You're welcome to download a pdf copy by clicking the link above or the image below.
Here's the introduction to whet your appetite.
"We’ve grown up being told that when we assess tree risk we should be looking out for tree ‘defects’. The problem with this approach is what are commonly labelled as defects often aren’t defects at all."
Any publication about tree risk management lacks credibility if it neglects the overall risk. That's because the overall risk from branches and trees falling and causing death, injury, or property damage provides the 'Context' (ISO 31000 - Risk Management). It gives us a base rate.
This is the context of the overall risk in VALID's Tree Risk-Benefit Management Strategies.
"Compared to other everyday risks we readily accept, the overall risk to us from branches or trees falling is extremely low. Our annual risk of being killed or seriously injured is less than one in a million. That's so low, we're at greater risk from a 200 miles (320km) round trip drive to visit friends for a weekend than from branches or trees falling for a whole year. Given the number of trees we live with, and how many of us pass them daily, being killed or injured by a tree is a rare event; one that usually happens during severe weather."
Why is establishing context and base rate so important? A risk expert nails it.
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