The impact factor, a numerical score that claims to rank the importance of scientific journals, may be resulting in unnecessary pressure on researchers to publish in journals with high values for that score.

With some qualifications, the impact factor is a measure of the average number of citations for papers published in a particular journal. It is calculated by counting the total number of citations papers in the journal receive, and dividing by the number of papers published in the journal. These statistics are compiled by the Institute for Scientific Information (ISI).

Does the impact factor provide an accurate measure of a journal’s importance? In counting citations, only papers published in the past two years are considered, though many research papers may be influential for much longer than two years. Also, items such as news articles and editorials that some journals publish are not counted in the denominator of the impact factor, but citations to those news articles may be included in the numerator, inflating the impact factor of journals that publish those types of articles.

Review articles, such as those published in Reviews of Modern Physics, are often much more highly cited than the average original research paper, so the impact factor of review journals can be quite high.

In some fields, there have been reports of journals that have raised their impact factors by such tactics as adding news articles, accepting papers preferentially that are likely to raise the journal’s impact factor, or even asking authors to add citations to other articles in the journal.

APS journals have not been much affected by these types of problems, said Martin Blume, APS Editor-in-Chief. In fact, Blume says he makes a point of trying not to pay attention to the impact factor.

Blume and others are more concerned that in some cases hiring and tenure committees or funding agencies may use the impact factor inappropriately as a way to evaluate individual researchers. “There is no quantitative metric of excellence. High impact factor journal publication is not a measure of excellence of the individual,” said Blume.

Ivan Schuller of UCSD says he likes to publish in the Physical Review journals, because he wants his work to be read by physicists. But some of his students feel that publishing in Physical Review instead of Science or Nature, which have higher impact factors, puts them at a disadvantage when applying for jobs. They believe some universities may simply look at the impact factors of journals they’ve published in, rather than carefully review the individual’s work.

Paul Kwiat of the University of Illinois recently co-authored a paper on quantum computation that was published in Nature. But the impact factor, which Kwiat had never heard of, wasn’t considered in the decision of where to publish.

"We chose Nature because we thought we had an item that might have some general public interest, while being novel science," Kwiat said. "I'm not sure I know any kind of quantitative 'impact factor', but surely scientists know that some journals are more prestigious than others, partly in view of the difficulty of getting published in them."