If you are researching tree risk assessment in South East Queensland you will run into three names: QTRA, TRAQ and VALID. They are aiming at the same job — deciding whether a tree presents an acceptable level of risk — but they work differently enough that the choice changes what your report can be used for.
The three methods in one paragraph each
QTRA — Quantified Tree Risk Assessment
QTRA produces a number: an annual probability that a given tree will cause harm. The assessor selects a range for three inputs — the target (what is beneath the tree and how often it is occupied), the size of the part that could fail, and the probability of that part failing within twelve months — and the system combines them, using Monte Carlo simulation, into a Risk of Harm expressed as something like 1 in 45,000. That figure is then read against published tolerability thresholds.
TRAQ — Tree Risk Assessment Qualification
TRAQ is the ISA's method, delivered in Australia through Arboriculture Australia. It produces a category: low, moderate, high or extreme. The assessor works through a structured matrix combining likelihood of failure, likelihood of impacting a target, and consequences of failure. It is the most widely held tree risk credential in Australia by number of practitioners.
VALID
VALID is the newest of the three and takes a probabilistic approach built around the assessor's own judgement, calibrated against a reference set. It is designed to be quick in the field and to make the assessor's confidence explicit.
The difference that actually matters
It is not accuracy. A 2023 study in Arboriculture & Urban Forestry compared 28 TRAQ-trained arborists, 21 QTRA-trained arborists and 11 arborists with no formal risk training, and found no statistically significant difference in variability between the methods (minimum p-value 0.251). Whichever system you use, two competent assessors can reach different conclusions about the same tree.
Two other findings from that study are more useful. Untrained arborists recommended removal roughly 50% of the time — far more often than trained assessors under any method. And QTRA-trained participants were more likely to recommend monitoring than their TRAQ-trained peers, rather than intervention.
The largest single variable in whether your tree gets removed is not which method the assessor uses. It is whether the assessor has formal risk training at all — and, separately, whether they profit from the removal. Neither of those appears in the method name.
Where the numeric output earns its keep
A category is a conclusion. A number is an argument. The distinction matters in four situations that come up constantly in SE Queensland:
- Defending a decision to retain. If a tree later fails, “we assessed it as moderate” is weak. “We calculated a risk of harm of 1 in 250,000, below the threshold at which intervention is indicated, and re-inspected annually” is a documented, reasoned position.
- Prioritising a budget across many trees. Thirty trees rated “moderate” give you no ordering. Thirty numeric risks sort themselves.
- Council applications. Where a protected tree is involved, a quantified figure demonstrates that removal was not the first resort.
- Insurance and legal. Both deal in probability. A number translates; a category has to be explained.
Where the categorical output is fine
For a single tree over a rarely-used corner of a private garden, a TRAQ assessment from a competent assessor answers the question perfectly well, and there is no advantage in a decimal place. Method snobbery is not useful. The report should match the decision being made.
| QTRA | TRAQ | VALID | |
|---|---|---|---|
| Output | Numeric annual probability | Category (low–extreme) | Probabilistic, confidence-explicit |
| Inputs | Target, size, probability of failure | Likelihood of failure, of impact, consequences | Calibrated judgement |
| Best for | Legal, insurance, multi-tree prioritisation, defending retention | General amenity assessment, widely recognised | Rapid field triage |
| Practitioners in Australia | Fewer | Most common | Growing |
A note on “arborist” versus the method
“Arborist” is not a protected or licensed title in Australia. It tells you nothing about risk training. The credential to ask about is the risk method — QTRA, TRAQ or VALID — and whether the person holds a current certification in it. It is a fair question and any competent assessor will answer it without hesitation.