The academic vocabulary can be viewed in ascending or descending order of frequency. The ranking is based on the number of hits returned by Yahoo† on a September 2008 search restricted to sites in the educational domain (.edu). The number shown is in millions, so the word add, for example, got 269 x 1,000,000 = 269,000,000 hits.
It is important to note that the search was done on the keyword only; the frequency ranking, however, results from multiplying the search hits number by a factor* representing the alternative forms of the keyword.
Furthermore, no attempt was made to filter out hits that did not correspond to the meaning of the word chosen for this list. Thus, the million+ hits for the word stable include not only those with the chosen meaning not changing; not easy to change (adjective) but also many with the meaning of the dwelling of a horse (noun).
For these reasons the ranking numbers themselves are not particularly significant and contain some anomalies. They do, however, give a reasonable indication of the relative importance to learners of the words listed. All other things being equal, it is preferable to learn more common words such as available and access before less common ones such as distort and expel.
† Yahoo has been found to provide more consistently reliable results for large numbers of hits than Google. [See the research by French linguist Jean Véronis.]
* The Yahoo hit count has been multiplied by a factor according to the class (part of speech) of the word. The factor for verbs is 3.0 to account for the additional verb forms (add: adds, added, adding). The factor for nouns is 2.0 to account for the plural form; and the factor for adjectives/adverbs is 1.5.