The lack of coverage of monographs - the main output of most humanities researchers - by prominent citation databases is often highlighted as a bar to the systematic use of bibliometric data in assessment exercises such as the UK's research excellence framework. But Thomson Reuters' Book Citation Index, part of its Web of Knowledge, will cover 25,000 of the "most significant" academic books from disciplines including the humanities dating back to 2005. Another 10,000 will be added each year.

Michael Jubb, director of the Research Information Network, said he was unclear how the books were selected and whether the database would incorporate older works as it expanded. But he welcomed the fact that books and articles would now be cross-referenced.

"There is the increasing risk that books will become invisible in a networked environment, and getting academic books into (databases) such as Web of Knowledge so that they become more visible will be a very good thing," he said.

Patrick Dunleavy, professor of political science and public policy at the London School of Economics, said the index was an example of how quickly the field of citation analysis was developing. He said it further highlighted the Higher Education Funding Council for England's "extreme oddness" in having already ruled out the systematic use of citation data in the 2014 REF.

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Some REF panels, mostly in the sciences, will look at citation data provided by SciVerse Scopus, but all are prohibited from using them as substitutes for reading submitted outputs.

"By 2014, every academic's basic publication record will have citation data on it and there will be a huge credibility gap for Hefce and the panels if they don't get real," Professor Dunleavy said.

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He added that Google Scholar indexed many more books than the 25,000 in the Thomson Reuters database, and he looked forward to the impending release of the former's citation aggregation tool.

paul.jump@tsleducation.com.