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What are baselines? Baselines are measures of cumulative citation frequency
across large groups of papers that provide expected citation rates for
groups or tiers of papers. Since citation frequency is highly skewed,
with many infrequently cited papers and relatively few highly cited papers,
average citation rates should not be interpreted
as representing the central tendency of the distribution, but rather as
guidelines or benchmarks. Similarly, percentiles,
or other fixed percentage cuts, indicate the citation rates for specific
top segments of the citation distribution.
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.
Time period for counts:
The count period for baselines is 10 years,
plus partial-year counts
for the current year (data is updated six times a year). For the all-years
counts any papers in the 10+ year period can be cited by any items in
that same period. For individual year counts citations are cumulated
from the beginning year to the end of the 10+ year period. Thomson Scientific database
years are used to define the time periods, that is, when items entered
the Thomson Scientific database.
 Average
Citation Rates
Average citation rates are calculated for each year of the
10-year period,
based on a culmination of citations from the year of publication to the
current year. (Averages are calculated by adding up the citation counts
of individual papers and dividing by the number of papers.) An average
for the full 10-year period is also given in "All Years." Rates are given
for individual fields or all fields combined. An average of 10.3 for physics
in 1991 means that on average, papers in physics journals were cited 10.3
time from 1991 to the present. Ten-year averages for each field (or all
fields) can be used as baselines for the citations per paper values given
in the scientist, institution, country, and journal rankings, provided
that the entity published papers over the same 10-year period. Field averages
for individual years can be used to compare the performance of individual
papers published in the given year, whether those papers are among the
highly cited papers listed in Essential Science Indicators or papers from the Web of Science.
Percentiles
The term "percentile" denotes a citation threshold at or above which a
fixed fraction of papers fall. Usually meaning a 1 percent cut, the term
percentile is used here to denote any fixed fraction of top papers ordered
by citation count. The levels we have selected for listing by field and
year are 0.01%, 0.1%, 1.0%, and 10%.
More information about thresholds.
The distribution of citations over papers is highly skewed, approximating
a power law distribution, with relatively many infrequently cited papers
and few highly cited papers. One method for making a selection is to rank
papers in descending order by citation frequency, and select the top fraction
of papers. The percentile table shows the citation count threshold for
four different percentile cuts for each field and year, as well as all
fields. For example, a threshold of 44 citations for 1993 papers in astrophysics
will select about 1 percent of the 1993 papers in the astrophysics journal
set.
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cites
Data Information:
