Field testing of wood and wood-based products is so far the most reliable way to get information on performance with respect to durability and expected service life. However, field trials are extremely time and cost consuming. The main objective of the present study was to investigate possible correlations between early stages of decay, preferably during the first 5 years, and the actual service life of a wood material. The hypothesis was that such correlations could be used when developing realistic models for service life prediction of wood materials in different applications. Data from Nordic field trials with more than 20 000 stakes of untreated and preservative-treated wood has been re-analysed and evaluated according to new approaches. The evaluation showed that the average life of stakes in test can be predicted with high precision by using the median to rating 4 (failure) as the basis for the determination. By this method, a value of the average life will be obtained in a shorter time than if one must to wait until the last stake in a series has failed. Further examination of data aimed at finding out how the time to early stage decay correlates with the average life. The approach was to analyze the time in testing needed for the development of decay corresponding to a rating of 1, 2, 3 and 4 according to EN 252. The evaluation also included analysis of the time needed for 5%, 25% and 50% of the stakes in a test to reach the rating of interest. Results clearly showed that the early stages of decay are reflected in the long time performance of the stakes. However, the overall conclusion is that the reliability of predicting the long term performance of wood is lower the earlier in the test the information is extracted.

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