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In visiting more than a hundred special libraries and information units in Britain (not to use their resources but to study their methods) I have often been concerned by the inadequacy of the private indexing systems in use, although there were many exceptions. The classifications were often crude, and/or outdated, and/or quite insufficiently detailed, and were seldom provided with good subject indexes. Alphabetical subject‐heading indexes are more difficult to assess on a brief visit, but these were seldom detailed enough, so that fifty or more documents could be indexed under one heading. Seldom was there a separate subject‐headings list, and often no recognition of the need of one. Guidance for the user was very often poor.
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Debbie Campbell (1963) studied this question.
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