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Journal Impact Factors are a form of measurement used to determine the relative standing of journals in particular fields. Journal Impact Factors are derived from an analysis of the data held on the ISI Citation Indices (which you may know as the ISI Web of Science database). These are published by the Institute of Scientific Information, Philadelphia.

A measure is made of the frequency with which articles in a journal have been cited. The impact factor for a journal is calculated by dividing the number of current citations to articles published in that journal in the two previous years by the total number of articles published in the journal in the two previous years. For example, for 1997 impact factors the following formula was used:

The impact factor will help you evaluate a journal’s relative importance, especially when you compare it to others in the same field.

Journal Impact factors can be accessed and compared through the Journal Citation Reports database (JCR).

JCR covers over 7000 journal titles from over 3000 publishers worldwide. The databases cover the most recent year of data available for impact factors (1998).

JCR lists journal titles and assigns an impact factor to each title. For instance Annual review of biochemistry might have an impact factor of 35.5 while Journal of clinical immunoassay might have a factor of 0.462. The higher the impact factor, the higher the perceived reputation of the journal. The impact factor for Journal of Clinical Immunoassay will have been calculated in a similar way to the example shown below:

Journal of Clinical Immunoassay	1995	1996	1997
citations to papers published in year	34	32	9
no. of papers published in year	71	72	68

Some Cautions When Using Impact Factors

The following points should be borne in mind when consulting impact factors:

citation does not automatically imply that a work is of high quality: a work may be heavily cited because lots of other authors are refuting the research findings it contains.
beware of citation bias: people may cite their own work, or work from the journals in which they publish.
an impact factor is a measure of average citation impact, not individual citation impact, so an impact factor cannot be used to measure the performance of an individual.
time needs to elapse before a meaningful citation analysis can be made, so new journals tend to fare badly.
not all research work is published and cited in the citation indices: conference proceedings, for example are often poorly covered.
there is a bias in favour of english language material on citation indices.
different fields of research publish at different rates: there is generally a much stronger culture of publishing in journals and citing the worh of peers in the biomedicine than in engineering.

Cited Half-Life

Another factor derived from citation indices is the cited half-life of the journal. In a subject like Computer Studies it is unlikely that people need to refer very often to articles published 10 years ago. In Geology it is more likely that old volumes are still used and quoted on a regular basis. In its subject listings JCR gives a cited half-life for each title.

For example, Journal of dairy science has a half-life of 7.5 years while Journal of dairy research (lower down the list for impact) has a shelf-life greater than 10 years. This would imply that the research published in Journal of Dairy Science has a high initial impact, which then declines quite sharply. Researchers would seem to go on quoting from articles from Journal of Dairy Research over a longer period of time.

The half-life represents the number of years of publication (back from the current) which account for 50% of references to each title.

Immediacy Index

This index tells you how quickly (on average) an article from a journal gets cited. For researchers working in cutting-edge research, it can give a useful perspective and enables journals to be compared for immediacy of impact.

Citation Indices

Citation indexing enables you to find papers which have cited a particular author in their references. Through citation searching you can follow the development of an idea or theory through the literature. For example, someone writing about evolution today might well make a reference to Darwin. If you were interested in seeing how Darwin's work has been used by other authors you can use citation searching to find the recent papers which refer to him.

In the context of RAE, citation searching can be used to find out the extent to which your own work has been cited by other authors. This will give some idea of the impact of your work on the broader research community.

Citation searching can be done on the Web of Science database; which covers the Science Citation Index, Social Science Citation Index and Arts and Humanities Citation Index. Each article in these databases includes a list of the references that are cited in the article. These can be searched to find patterns of citation.

When performing a citation search it is generally best to search for an author name. Type the surname followed by first initial as shown below:

sanderson a*

Use an asterisk (*) to find the largest number of possible alternatives for the author: they may have published papers as A. Sanderson, Andrew Sanderson, or even A.E. Sanderson if they have a middle initial. You can restrict to a particular year if you want - at the full search screen choose your year(s). This will find you, for example, all the papers published in 1999 that refer to the Sanderson's work, but not all the papers that refer to Sanderson's 1999 papers. You can restrict to Sanderson's 1999 papers by typing this date when you get to the cited search form (in the cited year box).

It makes no difference whether or not the author is listed as first author of the paper: Web of Science will find citations for all authors regardless of order listed.

Beware, also, of variations on names that may be used on publications for an author. For example: the author Tony Smith may have published papers as Tony Smith or as Anthony Smith (smith t or smith a).

For authors with common names, you may find lots of references; not all of which will be the appropriate person. It's worth checking the author's address if this happens. For example; if you are searching for R. Gee, you will find a number of authors based at Leeds, Michigan State University and the University of California. Only the Leeds author would be the one you were (presumably) looking for. You can check an author's address by following the link from the database to the reference for their paper.