Observer Variation as a Source of Error in Assessments of Crown Condition Through Time

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Data from the annual assessments of forest condition in Switzerland indicate that there are a number of problems with the collection of even the most basic information relating to forest condition. Of the 13 indices examined, 3 are assessed as continuous variables and 10 as categorical variables. In all cases, significant (P < 0.05) differences were found in 1 or more years between the field teams and the control team(s). For two measures of defoliation, these differences were present in all years. The assessment of the categorical variables was more reliable, but the differences indicate that the data must be treated with caution, even when only two or three relatively simple options are available to the surveyors. The surveyors involved in the work were all experienced foresters who had received a high level of training. The results suggest that many data related to forest measurement and assessment that have been reported in the literature cannot be reliably compared. The results also indicate an urgent need for the intensification of quality assurance work (training and subsequent checking) within the context of forest assessment. For. Sci. 41(2):235-254.

Keywords: Assessments of crown condition; confidence interval; hypothesis testing; observer error; statistical significance

Document Type: Journal Article

Affiliations: Swiss Federal Institute for Forest, Snow and Landscape Research, Z├╝rcherstrasse 111, CH-8903 Birmensdorf, Switzerland

Publication date: May 1, 1995

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