Impact of assessor on tree risk assessment ratings and prescribed mitigation measures

Tree risk assessment is an inherently human endeavor that can be influenced by risk perception, risk acceptance, and professional bias. Tree risk assessments from 296 arborists were evaluated to assess tree- and assessor-based factors that influenced ratings. Additionally, we investigated sources of variability associated with the main inputs of risk assessment - likelihood of impact ratings, likelihood of failure ratings, and consequences of failure ratings. Finally, we assessed the factors that influenced prescribed mitigation measures. Results indicate that professionals with training and industry credentials had lower risk ratings and were less likely to prescribe more active mitigation measures like tree removal. More notably, there was significant variability among raters, with the likelihood of impact and consequence of failure serving as the most variable factors in tree risk assessment.