280 Komen and Hodel: Field Precision of the CTLA Trunk Formula Method Watson (2001) suggested a method for improve- ment: the publication of databases of appraised tree values. These databases would not elimi- nate the need for a formula but would provide appraisers with established documented values to which they can compare their values. Watson provided tables of comparable appraised values for trees by species and size classes derived from more than 13,000 actual formula method appraisals, which help the appraiser to test the reasonableness of appraised values. The goal in this study was to compare and evaluate appraised values of several trees by sev- eral appraisers using the CTLA TFM. An analysis of the results of this study could help to identify problems of precision and/or methodology of this formula, if any, and suggest ways to improve it. To test the variability between appraisers, the study authors set up a study where indi- viduals could independently appraise the same trees without influencing each other’s results. Although the original intent of the study was to calculate the standard deviation of each of the measured and calculated values, the most impor- tant discovery was the identification of the four key elements of error that had not been specifically identified in previous research on tree apprais- als: personal value error, personal observation error, measurement error, and systematic error. Although the design of this study did not isolate these variables, the identification of these ele- ments should be the foundation of a future study. MATERIALS AND METHODS The authors used the same method Watson (2002) used for tree selection and data col- lection, but only gathered data for the CTLA method of appraisal and added the measure- ments of trunk circumference and species rating. It was felt that this method was an effec- tive way of collecting data without placing an undue burden on the study participants. In the study, the authors wanted to mimic the experience of the typical field appraisal, while still retaining control over tree selection, so as to allow comparisons between different appraisers. The local arboretum was chosen as the study loca- tion because it possessed a good variety of trees to choose from and good public access. The study ©2015 International Society of Arboriculture was conducted at the Los Angeles County Arbo- retum & Botanic Garden in Arcadia, California, U.S., between 15 July and 15 August 2014. Ten trees were located and identified, and 14 certified arborists were invited, independently, to appraise these trees according to their availability over the four-week period. Each tree was marked on a map, showing its location and its photograph for identification. The authors selected the trees to include a variety of health, location, and spe- cies ratings. Some trees were deliberately selected to include a trunk configuration that is not clearly defined according to the Guide for Plant Appraisal. The condition of the trees did not vary significantly over these four weeks. Each of the arborists were provided with a clipboard, a map, a form to fill out, a copy of the Guide for Plant Appraisal, and the same tape measure (30.48 m). Participating arborists were greeted individu- ally upon their arrival at the arboretum and were provided with the study materials. The authors gave a short introduction and overview to the study, explained their participation, duties, and responsibilities, and then observed each conduct- ing a practice appraisal on a tree separate from the 10 subject trees. Explicit instructions on how to conduct the appraisal were not provided; however, the authors did respond to questions pertaining to the proper completion of the data collection form. This was so the authors could avoid adding their own experimenter’s biases to the opinions of the participants. The par- ticipants were expected to appraise the trees the same way they do in their professional practice. Participants conducted their data collection independently, without guidance or influence. The authors could have conducted this study all at once, leading the participants to each tree together as a group, but this would have influenced their results because they would have observed each other looking at each sub- ject tree, making them more likely to see defects they would not have otherwise seen. Participants made their circumference mea- surements and subjective ratings for the species, location, and condition factors and recorded this data on the forms provided. The study authors provided the standard species ratings published by the local Western Chapter of the International
September 2015
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