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Countries vary in their level of scientific effort as measured by publication and citation activity. The distribution of effort is uneven, and dominated by the developed countries. The level of activity is roughly correlated with national GNP or other measures of economic output. Comparing countries on a citations per paper basis can help correct for size (output) differences.

Counting method for articles and citations:

Country counts are based on the institutional affiliations given on published papers. A paper is attributed to a country if the paper carries at least one address from that country. All addresses are considered, not only the address listed first. If a country appears more than once on a paper, it is counted only once for that country. All unique countries on a paper are credited equally for the paper. All citations received by a paper are credited equally to all the countries 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 the individual scientist, institution, or country.

Citation thresholds:

Countries selected for listing in Essential Science Indicators must meet a cumulative citation count threshold for the 10+ year period. The thresholds are set separately for each field. More information about the actual thresholds used in Essential Science Indicators.

The absence of a country's name in Essential Science Indicators means the country did not meet the citation threshold in any field.

Country Name changes and unification:

Country names have been unified to reflect name or country boundaries that have changed. For example, West Germany and East Germany have been unified to Germany for the 10-year period. The United Kingdom has been treated as a single country, rather than having separate entries for England, Scotland, Wales, and Northern Ireland. Data on some of the newly independent states of the former Soviet Union are available only after 1995, and for some eastern European countries since 1993.

Rank orderings:

Countries 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. The citations per paper ranking normalizes to some degree for country output.