Taking the Numberwang out of Tree Risk

VALID has four easy to understand traffic light coloured risk ratings, and this is where they sit in the Tolerability of Risk Framework (ToR).

The Tolerability of Risk Framework is an internationally recognised approach to making risk management decisions where the risk is imposed on the public.

The ToR triangle gets fatter and redder where more attention and resources should be allocated to managing the risk. It gets thinner and greener where less attention and resources should be allocated.

Where ToR is amber the risk is Tolerable if it’s ‘as low as reasonably practicable’ (ALARP) - where the costs of the risk reduction are much greater than the value of the risk reduction.

VALID has applied ToR to tree risk but has removed the numberwang because:

1) Tree risk has too much uncertainty to credibly measure at single figure accuracy with risks like 1/4, 1/300, 1/10 000, or 1/500 000 000.

2) Risk outputs as probabilities create friction in communication because many people struggle with numbers. Research shows that about 25-33% can't rank 1:10, 1:1000, and 1:100 risks from highest to lowest.

3) The risk assessor and duty holder are spared the complexity of numerical cost-benefit analysis in the amber ALARP zone.