 Difficult methodological issues arise in connection with the collection, annotation, and use of corpus data. For example, there is the issue of extremely rare expression tokens. Are they accurately recorded tokens of expression types that turn up only in consequence of sporadic errors and should be dismissed as irrelevant unless the topic of interest is performance errors? Are they due to errors in the compilation of the corpus itself, corresponding to neither accepted usage nor sporadic speech errors? Or are they perfectly grammatical but (for some extraneous reason) very rare, at least in that particular corpus?