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Thomson Scientific indexes the segment of the journal literature that exerts a disproportionate influence. The principle involved in this coverage strategy is based on a well-known concept in bibliometrics, Bradford's Law of Scattering. Bradford's Law asserts that a relatively small group of journals will account for the large majority of important and influential research in a given field.

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Papers appearing in the multidisciplinary science journals Science, Nature, and Proceedings of the National Academy of Sciences of the USA (PNAS) were reassigned to specific categories.

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The data may not be redistributed to any other party, resold in any form, or incorporated into derivative databases. Publication—in print or electronic form—of data residing in or compiled from Research Fronts is expressly prohibited without the written permission of Thomson Scientific.  

If you have questions on this licensing agreement, please contact the Research Services Group, Thomson Scientific, 3501 Market Street, Philadelphia, PA 19104, USA. E-mail: nancy.bayers@thomson.com (215-386-0100, x1776; fax: 215-387-1266); mary.vanallen@thomson.com (215-386-0100, x1885; fax: 215-387-1266); or ann.kushmerick@thomson.com (215-386-0100, x5351; fax: 215-387-1266).

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For the annual research front data, a single year (2005) of citing papers is used. The source papers, those being cited, can be from 1981-2005, spanning a 25 year period.

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Next, we compute the number of times pairs of highly cited papers have been cited together or co-cited. This defines the frequency of co-citation of the two highly cited papers. To do this, we set a threshold on the integer co-citation count to eliminate very low values. Then, we convert the remaining pairs to a normalized form using the following formula:

Normalized co-citation = Integer co-citation frequency of A and B/(citation frequency A*citation frequency B)^.5.

This is the so-called cosine coefficient of similarity where we divide the co-citation frequency by the square root of the product of the citation frequencies of the two papers. A second threshold is set on these normalized values. In this set of annual research fronts a threshold of 2 or more was used for co-citation counts, and the normalized threshold was set at 0.2.

Identifying research fronts involves manipulating the co-cited pairs in order to group together those highly cited papers that are strongly related. Starting with a co-cited pair that meets the two thresholds, a clustering procedure then finds other pairs that share common papers. The gathering or grouping process continues until no other pairs of papers can be added to the set at the selected thresholds. This process is commonly known as single-link clustering. As an additional filter, if the resulting cluster exceeds a size limit, the normalized co-citation threshold is increased until the large cluster is broken up into smaller constituent pieces none of which is greater than a maximum allowed size. The resulting clusters vary in size from a minimum of two papers to this maximum limit. In the current database, this size limit was set at 50 papers.

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