1.8% of tokens in the biomedical test set had POS tags like 'HYPHENATED', which are not part of the tagset for the training data, and were labeled incorrectly by all systems without access to labeled data from the biomedical domain.
an error rate of 1.8 + 3.9 = 5.7 serves as a reasonable lower bound for a system that has never seen labeled examples from the biomedical domain.