Sevim McCutcheon explores the strengths and weaknesses of keyword and controlled vocabulary searching with particular reference to the Library of Congress Subject Headings system, coming to the conclusion (as exemplified by the practice in her own university, Kent State, Ohio, USA) that each system has its strengths and weaknesses, that in practice they are complementary to one another, and that the best results are achieved by using the two in combination: in other words, they who have the most tools win.
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Sevim McCutcheon (2009) studied this question.