Suppose you have found one good paper for a report you are writing. What should you read next? You could follow that paper's references back into the past, or you could go the other way and move toward the future by finding newer studies that have cited it. The tool that links things in reverse in this way, starting from a cited paper and leading to the later papers that cite it, is the citation index.
Put simply, a citation index is ‘a list showing which later documents have cited a given document.’ Unlike an ordinary index that finds material by subject terms, it finds related research by following the chain of citations, and in doing so it greatly changed the course of library and information science and of academic search.
This article explains, in easy-to-understand terms, how the citation index came about and what it is, how to search by following citation links, the main metrics that grew out of citation indexes, and what to keep in mind when using the numbers.
How Citation Indexes Work and How to Use Them
What Is a Citation Index?
Surprisingly, the idea of the citation index came from the field of law. In the United States, Shepard's Citations has been published since 1873, recording which later court decisions cited a given case. In a legal profession that relies heavily on precedent, it was an essential tool for checking whether a ruling was still good law.
The American information scientist Eugene Garfield proposed applying this approach to the scientific literature. In a 1955 article in the journal Science, he argued that if the later papers citing each paper were collected together, researchers could find material by following the connections between ideas.
This vision became reality through the Institute for Scientific Information, which he founded. According to Garfield's own records, the first Science Citation Index came out in 1963 (the official history of Clarivate, which runs it today, gives the year as 1964), and it became the root of today's academic database Web of Science. In 2004, Google Scholar and Scopus appeared one after the other, and searching by following citations became a familiar method not only for researchers but also for students.
Three Ways to Search Through Citation Links
The greatest strength of a citation index is that it lets you find related research even if you do not know the right subject terms. Even when different scholars call the same concept by different names, their work is linked if they cite the same paper.
(1) Backward searching
Follow the reference list of the paper you are reading to find the earlier works that the research was built on. This shows you where an idea started.
(2) Forward searching
Use a citation index to find later studies that have cited the paper. This lets you see how the idea developed and what criticism it received.
(3) Finding co-cited works
If two papers are frequently cited together by many other papers, they are likely to be closely related in content. This is called ‘co-citation,’ and it is used to map out academic fields.
For example, if you start from the 1953 paper by James Watson and Francis Crick that revealed the double-helix structure of DNA, forward searching alone lets you trace decades of molecular biology research built on that discovery.
Key Metrics Born from Citation Indexes
As citation records piled up, people began counting them to gauge the influence of journals and researchers. The field that analyzes the literature quantitatively in this way is called ‘bibliometrics.’
- Citation count: The number of times a paper has been cited by other works. It is the simplest and most basic metric.
- Journal Impact Factor: The number of citations received this year by the papers a journal published in the previous two years, divided by the number of citable papers it published in that period. Garfield first devised it as a criterion for selecting journals.
- h-index: A metric proposed in 2005 by physicist Jorge Hirsch. It is the largest value of h such that at least h of a researcher's papers have each been cited at least h times.
For example, a researcher with an h-index of 10 has at least 10 papers that have each been cited 10 or more times. Because it reflects both the number of papers and the number of citations, it is widely used.
Four Things to Keep in Mind When Using Citation Metrics
Citation counts are convenient, but they are not a mirror that faithfully reflects the value of research.
(1) Citation cultures differ by field
The life sciences produce many papers and cite actively, while mathematics and the humanities have far fewer citations. Numbers from different fields should not be compared directly.
(2) Citations are not always praise
A paper cited in order to criticize flawed research still counts as one citation, just the same.
(3) The more famous, the more cited
In 1968, sociologist Robert Merton called the phenomenon in which already renowned scholars receive even more recognition the ‘Matthew effect.’ The same skew appears in citation counts.
(4) Do not judge individuals by journal metrics
The San Francisco Declaration on Research Assessment, drawn up in 2012, recommends against using journal impact factors to evaluate individual papers or researchers.
The citation index revealed the invisible bridges between papers and showed how knowledge connects and grows. The fact that we can now click ‘cited by’ in an academic search engine in a single step is also thanks to Garfield's vision.
Reading the flow of ideas that the numbers point to, rather than the numbers themselves: that is the first step toward using academic information wisely.
