Unanswered questions in the design of controlled vocabularies

The issue of free-text versus controlled vocabulary is examined in this article. The history of the issue, which is seen as beginning with the debate over title term indexing in the last century, is reviewed and then attention is turned to questions which have not been satisfactorily addressed by previous research. The point is made that these questions need to be answered if we are to design retrieval tools, such as thesauri, upon a rational basis.