Research metrics

The impact factor or journal impact factor (IF, JIF) is one of the most commonly quoted metrics. It is a relatively simple calculation, defined as the total number of citations in the past year which were of items published in the journal over the previous two years, divided by the number of "citable items" published in that time.

This is calculated for all journals in one of the core Web of Science citation indices, and is available through the Journal Citation Index service, which gives breakdowns of the calculation for each journal as well as historic data. The most recent year's data is also available through InCites.

A refreshed set of data is published by Clarivate every summer, using data from the previous calendar year - so the 2023 edition will cover citations from papers published in 2022, referring to papers published in 2020 and 2021.

Historically, the impact factor was one of the only widely available bibliometrics, and it became commonly used as shorthand for the quality or significance of a journal, and by extension used to indicate the importance of individual papers. However, this is potentially very misleading - it averages out any difference between individual papers. A high-IF journal could contain individual papers with very high citations as well as some with none at all - or, in rare cases, just one exceptionally highly cited paper that drives the average up.

We strongly recommend that you avoid using impact factors when judging the quality or significance of individual papers.

For any kind of internal assessment at UCL, this is one of the explicit requirements of the bibliometrics policy:

(7.) Ensure that metrics are applied at the correct scale of the subject of investigation, and do not apply aggregate level metrics to individual subjects, or vice versa. For example, do not assess the quality of an individual paper based on the impact factor of the journal in which it was published.

The impact factor can potentially have some value as a simple metric to indicate the "visibility" or "reach" of a journal - the more people read it, the more are likely to cite it - or as a proxy for identifying high-reputation journals - the most prestigious journals tend to have high impact factors - but even then you should be aware of the variation in the context of different subject areas. A journal with impact factor 5 would be about the top of the range in geology, but somewhere in the second quartile for oncology or cardiology. (Clarivate provide these quartile values for each category of journal, and they can give an approximate idea of where a journal sits within its field.)

For any work at UCL looking at individual papers, we recommend you consider using one of the more suitable normalised indicators instead. If you are being asked for impact factor information, for example as part of a grant application, and you have concerns about this, please feel free to contact us for advice.