Casebook 101 2.10.2011


It occurred to me recently that the chance of many people making the time to read an arborist’s report is remote. So I thought I’d take a tried and true approach and give the best bits to you in instalments. This instalment is about casebooks.


Newcastle City Council has been big on the use of ‘casebook history’ when it comes to tree failures and they have extrapolated this into a scary sentence like ‘The Laman Street trees will fail – we just don’t know when (so let’s take them out now)’.

There’s a 14MB document on NCC’s website (find your own link –  the claimed relevance to Laman Street is so circuitous I can’t bring myself to dumb you down by finding it) that shows some huge trees that have fallen over. Trees do that, unfortunately. As one of our arborists said to me, ‘I’ve never seen a tree that wasn’t potentially dangerous.’

Its use, the history of failures, is to help us understand why municipal tree staff don’t like big trees even though they will look you in the eye and talk about how much better large trees are than small trees when it comes to climate change and social benefit. Here’s Mark Hartley, one of the community’s arborists, on NCC’s casebook:

‘There have been repeated references to the ‘casebook’. Unfortunately, what has often been referred to as a casebook is, in fact, not a casebook but rather a log of failures. Whilst this is an important part of any casebook it is only one component and without the context of all the trees that didn’t fail it is, unfortunately, almost meaningless.

 ‘If, from a Risk of Harm perspective, we consider only Ficus hillii trees that have suffered catastrophic failures of their root plate in Newcastle in the last decade we find 4 risk related failures. To add some statistical depth to the casebook failures, if we assume that there are 360 Ficus hillii in Newcastle, this means, according to the casebook, that the historical failure rate of Ficus hillii in the last decade has been approximately 1 in 900.

 ‘Swain makes clear that he does not believe there is likely to be an increasing trend in the failures, and the casebook data supports this position . Swain states that “the recent failures in high wind and storms has removed those trees most likely to fail.” As has already been pointed out, the storms did not remove any of the original 17 trees in Laman Street. The trees were removed by a tree crew under the belief that the trees had moved and were now posing an unacceptable Risk of Harm. Regardless of the accuracy of this assumption, if these trees had moved in the 2007 winds then they were weaker than the other trees, and to that extent the implications of Swain’s statement remain valid. The trees that remained were the strongest trees at the time.

A friend told me these palms were planted from seed brought back from a war zone by Australian combatants

 ‘What the casebook actually reveals is almost the direct opposite of the argument that Newcastle City Council appears to have been arguing. Its very own records and casebook provide significant validation for the assertion that these trees do not pose an unacceptable Risk of Harm. The reason for this may be that, in maintaining failure records, the council team has been caught in the common trap of becoming fixated on the failures rather than looking at the data as a whole. As a result, the council has missed the bigger picture…

‘…Any case book needs to follow an appropriate statistical approach to avoid incorrect conclusions being drawn. Unfortunately, the casebook that has been regularly presented by the council has been so focused on failures, and in particular failures in one species of tree, that its value is meaningless without considering the species as a whole and perhaps other tree species as well.

 ‘Simply adding the relevant details about the numbers of trees in the population that haven’t failed provides a completely different, and more statistically valid, perspective.

 ‘With a fuller perspective, the casebook is a valuable tool that clearly shows that the probabilities of failure adopted by the council’s experts are unfounded and that the Probability of Failure provided in this and my earlier report are consistent with the observations and data collected by the council.’

The report is very educational – if you haven’t read it, you should. It’s here. At the time of writing, the link has been clicked on 130 times. Cool, don’t you think?

And yesterday’s Herald story was interesting, as usual – and the online comments are fun. One person thinks Save Our Figs ‘made Council’ spend $800 000 on this issue. Wouldn’t that much power be fantastic? I still look forward to the day that people have to leave their name when they leave a comment.  Home

I have fans everywhere


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