Number of citations and impact of publications

The number of a publication’s citations indicates how much it has been used in research since its publication. Citations can be seen as evidence of the usefulness of the publication to the academic community. 

Responsible use

• It is important to take into consideration the different publication and citation practices of disciplines as well as when a publication was published.
• Citations accumulate slowly. For the purposes of citation analysis, it is recommended that you wait at least two years after publication.
• Citation numbers depend on the database used, and the coverage of which varies.

The share of a unit’s publications among the most cited publications can be examined with the help of the Top x % share. Indicators are based on the citation distribution of databases: in databases, publications are grouped according to the year of publication, discipline and publication type, and citation distribution is determined for each group. The Top x% share indicates the share of the publication group examined that falls among the most cited. The aim is to enable comparison between different disciplines and periods.

Responsible use

• Indicators only take into account a small share of publications.
• The percentage limit can be adjusted (e.g., Top 1%, 5%, 10%, 15%).
• In recently established disciplines, citations can be scarce, which means that there is not yet any meaningful distribution.
• Different publication types accumulate citations in different ways.
• Differences in citation distributions can be substantial.
• The methods of calculation vary by database.

Where can I find Top x% indicators?

The citation impact of publications relative to other publications can be measured with the help of normalised citation indexes. The database generates groups of publications that have been published in the same year and represent the same discipline and publication type, and a citation average is calculated for each group. The number of citations to be examined is compared with the average number of citations for these publications. A normalised citation index indicates whether the publications examined have received on average more or fewer citations than corresponding publications. For example, the value 1.78 means that the number of citations is 78% higher than the average.

Example: A review article has received 15 citations. On average, review articles published in the same field in the same year have received 12 citations. Normalised citation index: 15/12 = 1.25. The review article has received 25% more citations than corresponding articles on average.

Responsible use

• The indicators are based on mean values. Individual publications with a high number of citations can have a major impact on the values produced by indicators. 
• When reviewing fairly recent publications, small changes in the number of citations can lead to major changes in the indicator value.
• Citation distributions are skewed; the mean and the median may differ.
• The methods of calculation vary by database.

The h-index combines citation impact and productivity in the same indicator. A researcher’s h-index value is h, if that number of their publications have received at least the same number of citations. An h-index value of 10 means that a researcher has 10 publications that have all at least 10 citations. 

The same h-index can be generated with very different publication profiles.

• The h-index does not take into account the date, discipline or type of the publication.
• The value depends on the database used and the scope varies.
• The index value cannot exceed the total number of publications.
• The index favours disciplines with large research groups and a large number of publications.
• Author roles are not specified in the index.
• Career duration increases the likelihood of a high value.
• Individual significant publications are not highlighted.

Indicators based on citations have been developed as tools for assessing journals, the most well-known of which is Impact Factor, which is calculated annually. The Impact Factor for 2024 shows how many articles published in the previous two years (2022–2023) have received citations, on average, in 2024. The citation data are obtained from the Web of Science database.

Responsible use

• Impact Factor measures the citation impact of journals in the short term.
• Impact Factor cannot be used to estimate the citation number of individual articles. This is due to skewed citation distributions.
• A low Impact Factor value does not mean that individual articles are of poor quality or less interesting.
• A low Impact Factor value may be caused by a narrow field of specialisation, a discipline citing older references, database coverage or a journal's brief publication history.