Linguistics
The AHD broke ground among dictionaries by using corpus linguistics for compiling word-frequencies and other information. It took the innovative step of combining prescriptive information (how language should be used) and descriptive information (how it actually is used). The descriptive information was derived from actual texts.
Citations were based on a million-word, three-line citation database prepared by Brown University linguist Henry Kucera.
Read more about this topic: The American Heritage Dictionary Of The English Language