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In ISI Web of Knowledge Journal Citation Report, we can find any journal Impact Factor (IF) and 5-Years Impact Factor (IF5), which is, as far as I get it, the Impact Factor over the last 5 years (well, the name is quite self-descriptive...).

What interests me is the difference between both indexes. I was wondering how to analyse this difference. If IF > IF5, does it means the journal IF is globally increasing?

And which one is more accurate while evaluating a journal?

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The impact factor is calculated over the two previous years (the 2013 number is based on referencing for 2011 and 2012). A high IF indicates that many papers are referenced very quickly (within 2 years of publication). This indicates high turnover rates for the journal. If the longer term value is lower, it could mean what you describe but can also mean that work published has very limited life span. In many fields where publications are slow to produce, in my case they may involve field and longer term experimental work, it is hard to gain many references very quickly and hence the longer term indicator becomes more important and higher. This is also true for small fields where the number of publications is too small for a regular IF to make sense. In field that I am familiar with the five year factor is higher than the ordinary IF bit it will likely vary depending mostly on the nature of the field; if publications have a long or short time span.

There is another factor called the Cited half-life (The median age of the articles that were cited in a year) which in my field often exceeds 10 years indicating the longevity of publications.

For further reading look at Journal Citation Reports

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How to compare impact factor over time

If you want to see whether the impact factor of a journal is increasing, then you should look at how the same metric has changed over time. You could choose the 2 year or 5 year impact factor for various reasons. However, you need to focus on only one of these metrics at a time.

The scimagojr website has similar data that is publicly available based on slightly different sources to the ISI data, but it provides information on how indicators have changed from year to year. E.g., Cites per doc

Here's one example for a journal in my field: http://www.scimagojr.com/journalsearch.php?q=30073&tip=sid

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What does a difference between 5 and 2 year impact factor mean

It's mostly caused by how citations to the journal are distributed over time. The citation half-life is a useful summary of this distribution. So, for example a citation half-life of 5 years means that half of all the citations that an article will obtain will be accrued 5 years after publication. The ratio of the 5 year to two year impact factor will be larger for journals with longer citation half-lives.

Importantly, fields vary dramatically in citation half-lives. Medicine, neuroscience, and nursing have short half lives, whereas psychology, mathematics, and the social sciences have longer half lives. So, using longer time windows for impact factor calculation will reduce differences between disciplines, although there are other metrics that can used if you wish to reduce discipline-specific bias.

Of course, it may also be that the underlying impact factor of the journal is increasing or decreasing over time, and this will also contribute (typically a small amount) to the nature of any difference between 5 and 2 year impact factor. Specifically, if the impact of the journal is increasing over time, the ratio of 5 to 2 year impact factor will be reduced.

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