Impact Factor is a measure of importance of scientific journals. It is calculated each year by the Institute for Scientific Information for those journals which it tracks, and are published in the Journal Citation Report. Impact Factors have a huge, but controversial, influence on the way published scientific research is perceived and evaluated.

Calculation

The Impact Factor is generally calculated based on a 3 year period. For example, the 2004 Impact factor for a journal would be calculated as follows:

A = Number of times articles published in 2001-2 were cited in tracked journals during 2003

B = Number of articles published in 2001-2

2003 Impact Factor = A/B

There are some nuances to this: ISI exclude certain article types (e.g. news items, correspondence, errata) from the denominator. Also, for new journals, ISI will sometimes calculate an Impact Factor with just 2 years of data.

Pros and Cons

It is sometimes useful to be able to compare different journals and research groups. For example, a sponsor of scientific research might wish to compare the results to assess the productivity of its investments. An objective measure of the importance of different publications is then required and the impact factor is one of such. Consequently, there is a demand for measures like this.

The favorable properties of the impact factor includes:

• Wide international coverage with more than 8400 journals from 60 countries.
• Results are publicly and freely available
• Easy to use and understand

Impact factors are not, however, infallible. It is, for example, questionable if the number of citations measures the quality or sheer amount of publications. Furthermore, in a journal which has long publication times, it might be impossible to cite articles new enough to fall inside the three year window. Indeed, for some journals, the time between submission and publication can be over two years, which leaves less then a year for citation. On the other hand, a longer temporal window would be slow to adjust to trend changes.

The most commonly mentioned faults of the impact factor include:

• Number of citations does not really measure quality of publication, but also quantity of publications.
• The temporal window for citation is too short. Classical articles are cited frequently even after several decades.
• The nature of results in different research areas yield different amounts of publications and at different pace, which has an effect on impact factors. Generally, for example, medical journals have higher impact factors than mathematical journals.

External links

• A list of Journal Impact Factors from recent years