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Defoliation (crown thinning) is widely used as a rapid method of tree condition assessment. As a method that is based on subjective visual observation it might be influenced by statistically significant observer bias. Significant observer bias has been discovere in some countries. We analyzed the significance of observer bias occurring in the Finland's forest condition monitoring system. We analysed the data of three training courses held to the field personnel (2006, 2007, 2008). Our results indicate that some inconsistencies occur between observers, but these are still not systematic in nature. In conclusion, the detected observer biases are independent incidents, caused mainly by the observer perception during the single events. Therefore there is no need to make any systematic corrections for Finnish national visual tree defoliation assessments. We suggest that the best way to improve field assessments is the proper education and guidance of field personnel.