Assessment of likelihood of failure using limited visual, basic, and advanced assessment techniques

Tree risk assessments can be categorized by their time, training, and equipment requirements. In arboriculture and urban forestry, practitioners must select a risk assessment method that is appropriate for the tree or trees to be assessed, available resources, and management objectives. While more detailed advanced risk assessment levels are believed to provide more accurate information regarding likelihood of failure (a component of tree risk), it is not clear how this additional information influences risk ratings. For this experiment we compared likelihood of failure ratings for trees assessed by 70 arborists using Level 1 (limited visual), Level 2 (basic), and Level 3 (advanced) risk assessment methods. Mean ratings did differ by level of assessment (P < 0.001). Mean likelihood of failure ratings for limited visual assessments were lower than the basic and advanced assessment techniques used. However, the differences between the basic and advanced assessment methods tested were less pronounced. Additionally, no level of assessment consistently reduced variability in ratings among arborists.