The term secondary services applies to current bibliographies, list of indexes and abstracts which are regularly published either in hard copy or as machine-readable database (say on CD-ROM or online through the net). These are called secondary services because they list the output of research published in the form of 'papers' in conference proceedings, journals, websites, eprint journals and full text databases. Examples of secondary services are Physics Abstracts (hard copy), INSPEC (machine-readable), Current Contents, Citation Indexes, Indian Science Abstracts (ISA), Current Literature on Science of Science (CLOSS), Social Science Index, etc. There are now thousands of secondary services on different subjects or topics. The aim of these services is to help in information retrieval, to find important and necessary items for study. The bibliography-type services like Physics Abstracts or INSPEC, BIOSYS, MEDLARS, ISA, etc. provide access to older materials (published in the immediate past). But services like citation indexes provide information about most recent items in relation to known information about past items (without any limit to the age).
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