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Generally, citations to papers peak in the second, third, or fourth year after publication, but some papers continue to be cited for many years. A few papers can exhibit delayed recognition. The patterns can vary greatly depending on the type of paper, the field, and the nature of the finding reported. Papers reporting discoveries, for example, can rise quickly and then fall as the discovery is further elaborated in other articles. Papers reporting methods or techniques can gradually increase in citation frequency over several years as the methods diffuse throughout the community and prove their utility.

Selecting highly cited papers:

Since citation rates vary by field and older papers are cited more than recent papers, the selection procedure for highly cited papers takes these factors into account. The first step is to count the number of papers cited at different levels of citation and construct distributions for each field and year. These distributions for each field/year are then used to set selection thresholds by taking the same fraction of papers.

Time period for counts:

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. Citations from all sources are counted, and are cumulated from the year of publication through the current year. 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.

Selection criteria:

Citation cutoffs specific to field and year are applied to all papers in the journal set to select highly cited papers. Citation thresholds are based on the distribution of citations, picking the specified top fraction of papers for each year and field. More information about the actual thresholds used in Essential Science Indicators.

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.