Visual inspection is often a crucial means of judging the efficacy and performance of a preservative. It is used in non-destructive testing in long term exposure trials or simply, when the practical use and precision of the human eye is a good means of measure. As for the procedure described in EN 252, the impression of softness due to decay of the wood has also to be considered. Although, a thumb and an eye are unequivocally refined sensors, there is no method published for their calibration according to the method they are used in. So, quality assurance of visual inspection is often neglected. This paper proposes a procedure that may lead to constructive discussion and harmonisation of the interpretation of rating scales among evaluators. It is based on the experience of European countries within the Nordic Wood Preservation Council (NWPC) and CEN/TC38 WG 25 working on the clarification and correct interpretation of rating scales as in EN 252. During three workshops, practical work was performed on wood stakes from the field showing different levels of attack by fungi. In average all evaluators agreed on the evaluation in 21% of the cases, while at least one evaluator disagreed in 67%. In 12% of the cases the stake was given three different ratings. A following discussion led, in almost every case, to agreement on a rating class according to EN 252. We suggest that especially accredited laboratories that perform this type of testing regularly participate in such workshops to "calibrate" themselves with their colleagues and the intention of the standards. A procedure, as described in this paper, for performing quality improvement of the evaluation according to the rating scale could be useful as an informative annex in all standards where visual inspection is used, e.g. EN 252, EN 330 and CEN/TC 12037.

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