The impact factor was first developed for comparing journals. It is still the oldest and most used indicator for measuring the impact of journals. Clarivate Analytics has the exclusive rights to the impact factor and the IF-numbers are only available in the Web of Knowledge Journal Citation Reports (JCR) database, which is updated yearly.
The impact factor is calculated as follows:
The amount of times a journal’s articles have been cited over a period of two years is divided by the amount of articles published in the journal during this time
There are many restrictions when it comes to the use of the impact factor, and the indicators have been criticised e.g. for the following reasons:
- The impact factors on different disciplines are not comparable!
- The impact factor favour such disciplines, where the publication rate is fast and where new articles are reacted upon quickly -> The impact factor-value of a journal should always be compared to citation coefficients of journals of the same discipline. IF is best suited for science and medicine.
- Journals that publish many review articles usually have higher IF-values than those that only publish a few articles. Disciplines that are broad and common are likely to have more articles with many citations than narrower disciplines (this is why journals in specialised disciplines have low IF-numbers)
- When calculating the IF-values, only the journal citations in JCR-database are taken into account. JCR includes about 10 000 scientific peer evaluated journals of which 2650 are of sociology disciplines. Thus, citations of other journals and monographic publications are not included in the IF-value calculations, instead self-citations of journals are taken into account
- Thus, it is better to favour IF-values of a longer perspective than two years (e.g. a 5 year impact factor)
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