Essential Science IndicatorsSM
Data Information:
Understanding Core Data - Baselines & Percentiles
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
Reuters-indexed journal articles or papers are counted.
Journals included:
Essential Science Indicators counts are based on an
Thomson Reuters 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 Reuters database years are used to define
the time periods, that is, when items entered the Thomson Reuters
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.
Should you have further questions, please
contact us.
