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essential facts

Essential Science Indicators is a compilation of statistical information (publication, citation, and cites-per-paper counts) for scientists, institutions, countries, and journals. It is based on 10 years of Thomson Scientific data. The Essential Science Indicators Data Information Pages will help you understand how Essential Science Indicators works using counting methods for articles and citations, time periods for counts, types of items counted, journals included, citation thresholds, name conflation, name variations, and rank orderings.

Essential Science Indicators, accessible by subscribers through a Web interface, is updated every two months. During the course of a year, the data series presented covers 10 years plus a successive number of recent two-month periods, eventually reaching an 11-year time span. At the end of the year, the compilation reverts to a 10-year data set, dropping off the oldest year of the series. 

Here are some rough statistics on the magnitude of the Thomson Scientific data file that is processed to obtain the information included in Essential Science Indicators:

In a recent 10-year period Thomson Scientific recorded about 9 million articles, notes, and reviews, published in roughly 9,000 indexed journals. Essential Science Indicators categorizes these journals into 22 broad disciplines. Each journal is assigned to one of the 22 disciplines (See complete journal list for Essential Science Indicators). Similarly, Essential Science Indicators then assigns each paper to a discipline—and only one discipline—based on the journal in which it appears. In the case of multidisciplinary journals, special processing is carried out to assign individual papers to fields based on the predominate field of the papers' citations and references. The number of citations received by these 7 million items, (originating, of course, from the same Thomson Scientific-indexed journal literature), is roughly 53 million. These are the footnotes, or references, that appear in the journal articles. The Thomson Scientific data file is unique in capturing these so-called cited references (see also: Classification of Papers in Multidisciplinary Journals).

Essential Science Indicators identifies the "essential core" of journal articles, scientists, institutions, countries, and journals from this large data corpus by setting selection criteria (a certain number of citations) for each of the disciplines. These thresholds, set to select some constant fraction of items, are described in an accompanying document (citation thresholds). For example, for highly cited papers, Essential Science Indicators selects the top 1% of articles by total citations in each annual cohort from each of the 22 disciplines. Highly cited papers in Essential Science Indicators total about 76,000 items. Essential Science Indicators also identifies "hot papers," which date from the last two years and which have received an unusually high number of citations during the most recent two-month period. About 1,500 hot papers are selected, representing the top 0.1% in the two-year period.

Of the roughly 3 million scientists' names appearing in the 10 years of Thomson Scientific data surveyed, about 50,000 are listed in Essential Science Indicators. This represents the top 1% of authors in terms of total citations in each of the disciplines over the 10 years. Each scientist name appears, on average, in 1.2 disciplines. About half a million institutional affiliations are scanned in the 10-year data file, and about 3,000 of these are selected for Essential Science Indicators, also representing the top 1% in each discipline (unification of institutional names is undertaken to obtain more accurate statistics). Each of the selected institutions appears, on average, in 3.1 disciplines.

For countries, about 150 are selected out of about 200 scanned, and for journals about 4,500 of the 9,000, both representing the top 50% by discipline and total citations over the 10-year period. As noted before, journals are assigned uniquely to only one discipline (with the exception of multidisciplinary journals), but each country appears on average in about 13 disciplines.

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