Citations make connections between scientific publications: the publications that authors cite make the connection between their current research and previous works and publications in scientific literature.
Citation databases index these references. That way you can look at what previously published works the author has based his research on, or you can look forward to what more recent research has been done based on the publication.
Citation analysis can be used as a tool to evaluate scientific achievements.
The most common scores within the citation analysis are the Impact Factor and the Hirsch index.
To make a citation analysis, using a database or a search engine with bibliographic references is required. The possibilities of citation vary greatly from one database / search engine to another. The most extensive functionality is in the Journal Citation Reports(JCR) of the database Web of Science. Journals can be sorted within the JCR by discipline (Subject Category) and by impact factor. The relative impact factor can be calculated from the journal’s ranking. This ratio reflects the importance of a journal within a field of research.
Since 2011, Google Scholar has the integrated service Google Scholar Citations. This service is free and as an author you can create a Google Scholar Citations account. Google Scholar Citations states the citation counts (per publication) and the Hirsch index of an author.
The best known citation score is the Impact Factor (IF). The impact factor is a measure to indicate the relative prestige (importance) of a scientific journal. The more articles from a particular journal are cited in other journals, the higher the impact factor.
Thomson Reuters annually publishes a list of the impact factors calculated by them, which are published in its annual Journal Citation Reports.
The impact factor can be distorted by several factors. For example, many review journals have high impact factors: the articles in those journals are review articles on a particular subject, and they are disproportionately cited. Chemical Reviews, for example, has an impact factor of around 50. There is also a strong variation between different fields.
The level of the impact factor often says more about the size of a particular field than about the 'importance' of it, that is why it is mainly suitable for comparing similar journals in the same field.
The Hirsch Index is a citation score that says something about the author. The formula for the Hirsch Index is:
H publications of the author have been cited at least H times on a total of N publications of this author.
(Also: N minus H publications are quoted (per article) less than H times.)
With this score peaks in citation (on article level) of an author are compensated. A single article of an author that is cited extremely often, is put in perspective with (rarely or) never cited articles of this author. A Hirsch index of an author never drops, unless a publication is withdrawn.