The National Center for Education Statistics conducts surveys which require the coding of the respondent's major field of study. This paper presents a new system for the coding of major field of study. It operates on-line i a Computer Assisted Telephone Interview (CATI) environment and allows conversational checks to verify coding directly from the respondent. The system "learns" by maintaining a database of response/coding pairs which can be incorporated into its algorithm after supervisor review. This paper analyzes the effectiveness of this approach and database in coding major field of study for the Beginning Postsecondary Students Longitudinal Study Second Followup 1990-1994. (Contains 2 figures, 2 tables.) (Author)