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. Clarivate
database years (the actual years when items are entered into the
Clarivate database, which is not necessarily the publication
year) are used to define the time periods.
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
Reuters-indexed journal articles or papers are counted.
Essential Science Indicators counts are based on an
Clarivate 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
would be assigned to specific fields. This should be taken into
account when analyzing the field ranking of an individual
scientist, institution, or country.
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