Arboriculture & Urban Forestry 44(2): March 2018 and NPV. The variables with high percentage agree- ment include Genus, DBH within 2.54 cm, and Tree Guards, as well as all Tree Problem sub-variables, except Root Problem: Sidewalk/Stones. Moderate agreement was found for Species, Tree Condition, Root Problem: Sidewalk/Stones, and Sidewalk Dam- age. Low percentage agreement was seen for exact DBH. Additionally, low PPV was seen for all pres- ence/absence variables except Stewardship, although NPV was high for all, except Sidewalk Damage. Table 3. Description of Sn, Sp, PPV, and NPV, and how they were calculated. Actual (staff) + Test (volunteer) + - True Positive (TP) False Negative (FN) Sn: The % of actual positives that test positive. SN =TN / (TP + FN) Sp: The % of actual negatives that test negative. Sp = TN / (FP + FN) PPV: The % of positive test results that are actual positives. PPV = TP / (TP + FP) NPV: The % of negative test results that are actual negatives. NPV = TN / (TN + RN) DISCUSSION TC2015 resulted in a spatially accurate data set that was incorporated into NYC Parks’ forestry opera- tional database. The high-quality spatial data were a particular success, indicating the scalability of the TreeKIT method. For street tree inventories, this spatial-positioning method with application- based data collection is highly recommended. This method not only produced more accurate spa- tial data than previous inventories, it also allowed for an ongoing QA process that greatly increased the quality of NYC Parks’ final spatial data set. In addition, the quality of volunteer data for (Genus, Species, DBH [within key tree variables 2.54 cm], and Tree Condition) had moderate-to- high agreement with staff data. Genus agreement (86.5%) with staff was higher than that for Species (77.6%). This was an expected result, as the study design called for greater focus on genus by includ- ing genus-only options, and instructing participants to focus primarily on genus-correctness, if species was unattainable. This study design was imple- mented because NYC Parks manages most species - False Positive (FP) True Negative (TN) 55 within a genus similarly, placing less importance on exact species identification. Overall, these results were similar to those noted by Roman et al. (2017), whose results found (mean) percentage agree- ments of 90.7% for Genus and 84.8% for Species, as compared to results of the current study of 86.5% and 77.6%, respectively. Additionally, agreement of 92.7% for DBH (within 2.54 cm), in the current study, was comparable to Roman et al.’s (2017) result of 93.3%, and percentage agreement for exact DBH was 32.0%, as compared to Roman et al.’s (2017) result of 20.2%. However, it should be noted that TC2015 measurements were collected as circum- ference to the nearest inch and converted to DBH, whereas Roman et al. (2017) collected sizes as DBH to the nearest tenth of an inch, making comparisons between the two studies tenuous. These DBH results suggest collection of circumference at breast height and conversion to diameter reduced confusion. NYC Parks learned some valuable lessons about volunteer engagement from TC2015. On the basis of recruitment and volunteer participa- tion, the data-collection events were very suc- cessful, with over 75% of citizen scientists having participated in the effort exclusively at events. Anecdotally, the reason for this event-centric data collection trend appeared to be the social aspect of events; though it should be noted that events alone did not result in volunteer retention (≈50% of individuals participated for a single day/event). The spatial data quality results noted above suggest those who contributed ≤10 surveys (approximately one day) had markedly less accu- rate tree locations than those who contributed ≥25 surveys, suggesting single-time participants may decrease data quality. Presumably those with low contribution levels joined due to pass- ing interest or were motivated by one-off reward offerings, such as free tickets for attending one event. In one case, more than 300 participants from a partner group did not participate again after receiving their reward. The same pattern was not noted among the rest of the citizen sci- entist cohort in response to the TC2015 branded rewards, likely because such rewards took more effort to obtain. Overall, the top ten contributors in TC2015 collected data for 57,965 trees (equiv- alent to 26% of volunteer-contributed trees), with an average spatial data rejection rate of ©2018 International Society of Arboriculture
March 2018
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