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Studies of vocabulary size based on dictionary sampling have faced several methodological problems. These problems occur in trying to answer the following three questions: (I) How do we decide what to count as words? (2) How do we choose what words to test? (3) How do we test the chosen words? The present study attempts to overcome these problems and checks in several ways to see if the problems have been overcome. The results indicate that what were previously thought of as conservative estimates of vocabulary size are likely to be the most accurate. These estimates suggest that well-educated adult native speakers of English have a vocabulary of around 17,000 base words. This represents an acquisition rate of around two to three words per day.
Goulden et al. (Sat,) studied this question.