Readability
Several vocabulary lists have been published by researchers. These lists are based on samples of published texts in English, and (less often) samples of recorded spoken language. The lists differ slightly according to the sources chosen, but they are very reliable. The items listed may represent more than one actual word; they are lemmas. For instance the entry "be" contains within it the occurrences of "is", "was", "be" and "are". The top 100 lemmas account for 50% of all the words in the Oxford English Corpus.