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Publications are grouped in terms of their age, discipline/field and publication type. A citation distribution for each group is formed by ordering publications based on their received citations. A mean value of citations is calculated for each distribution. Normalisation is based on the distibutions and mean values. The use of field-normalised indicators aims to allow for comparisons of publications accross disciplines. The sections below present these various field-normalised indicators.

Field-normalised

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Top x% indicators

The field-normalised Top% Top x% indicators are based on the database’s field-specific citation distributions. The Top% Top x% indicators show the proportion of the analyzed publication set that belongs to the most frequently cited x percentile of the publications of the same field, publication year and publication type. Top% Top x% indicators are used as indicators of high citation impact. For example, Top10% Top 10% share and Top1% Top 1% share demonstrate the relative share of publications reaching both high (10%) and top (1%) impact within the publication set.

Top x% -

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indices

The Top x% -index show the relative proportion of the analyzed publication set that belongs to the most frequently cited x percentile of the publications of the same field, publication year and publication type. For example, a Top 1% index value exceeding 1 means that more than 1% of the publications in the analysed publication set belong to the most cited 1% of the field of science in question.

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Responsible use

The Top x% indicators seek to account for disciplinary differences regarding publication and citation practices as well as accounting for the publication date. They are not sensitive to individual publications with a high number of citations.

Remember that

• Top% Top x% indicators only assign value to a small proportion of the publications, while others are ignored.
• Drawing the limit at a certain percentile is arbitrary; instead of Top 1% and Top 10% we could just as well analyse the Top 5% and Top 15%.
• The indicators are based on classifications of fields of science that are not entirely unproblematic (more information on this can be found in the chapter Classification of fields of science.
• When reviewing the Top x% indicators of fairly recent publications (published less than five years ago), it should be noted that the citation numbers are low in certain fields of science. In such cases, a meaningful distribution for calculating the Top x% indicator does not yet exist. 
• Different types of publications accumulate citations in different ways. A conference publication, for example, can become included in the Top x% publications with far fewer citations than a journal article.
• See also the questions related to assessing responsibility in the chapter Indicators.

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• The indicators are based on mean values. Individual publications with a high number of citations can have a major impact on the indicator’s value. 
• When reviewing fairly recent publications, it should be taken into account that the citation numbers are lower within certain fields, due to which small changes in the number of citations can lead to major changes in the indicator value.
• The indicators are based on classifications of fields of science that are not entirely unproblematic (more information on this in chapter Classification of scientific fields of science.
• See also the questions related to assessing responsibility in the chapter Indicators.

Normalised indicators in databases and other sources

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H-index

One of the best known bibliometric indicators is the h-index, also known as Hirsch index. It was developed by Jorge E. Hirsch in 2005. Read more about the topic in the original article. 

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