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Citation counts are a form of peer recognition and generally reflect the dependence of the scientific community on the work of individual scientists. It could be argued that highly cited scientists form the essential core of the scientific community. Many highly cited scientists have also received peer recognition in the form of honorific awards.

Counting method for articles and citations:

All authors on a paper are credited equally for the paper. All citations received by a paper are credited equally to all the authors on the cited paper. No restrictions are made on the citing items in compiling the citation counts, other than that they are recorded from Thomson Scientific-indexed journals only.

Time period for counts (cites, papers, cites per paper):

The time period for Essential Science Indicators counts is 10 years, plus partial year counts for the current year (data is updated six times a year). This means that any papers in the 10+ year period can be cited by any items in that same period. Thomson Scientific database years (the actual years when items are entered into the Thomson Scientific database, which is not necessarily the publication year) are used to define the time periods.

Types of items counted:

Papers are defined as regular scientific articles, review articles, proceedings papers, and research notes. Letters to the editor, correction notices, and abstracts are not counted. Only Thomson Scientific-indexed journal articles or papers are counted.

Journals included:

Essential Science Indicators counts are based on an Thomson Scientific journal set (see complete journal list for Essential Science Indicators) categorized into 22 broad fields. Fields are defined by a unique grouping of journals, with no journal being assigned to more than one field. The Multidisciplinary field contains journals such as Science and Nature which in an article-level classification would be assigned to specific fields. This should be taken into account when analyzing the field ranking of an individual scientist, institution, or country.

Citation thresholds:

Authors selected for listing in Essential Science Indicators must meet a cumulative citation count threshold for the 10+ year period. More information about the actual thresholds used in Essential Science Indicators.

The absence of an author name in Essential Science Indicators means the author did not meet the citation threshold in any field.

Name conflation:

Authors having the same last name and initials may represent multiple individuals. This is especially likely in the case of common surnames. The ability to break the name by field may to some degree disambiguate person X in field Y from person X in field Z; however, keep in mind that a listed name can still represent more than one author within the same field.

Name variations and variations in initials:

Individual authors may appear with or without middle initials, in which case two entries may need to be consulted. This is also true for authors who have changed their names, for example, from maiden to married names, or authors whose last names appear in multiple ways.

Rank orderings:

Scientists in the rankings are numbered from 1 to N, depending on the ranking option selected, with no repeated ranks for ties (ties are then ranked alphabetically). Ranking options include citations, papers, citations per paper, and alphabetic.