Last week I looked at citation histories for JGR-ES articles — most articles tend to accrue citations steadily through time after publication, and mean citations/year do not tend to show a strong decrease with time. So how does this relate to Journal Impact Factor (JIF), which is calculated by dividing the total number of citations received by a journal (for all articles) in the 2 previous years, by the total number of articles published in the previous two years. There is also a five year variant of the JIF. a time series of JIF for JGR-ES and other geomorphology journals can be seen here.
Specifically, what share of total citations to a JGR-ES paper are received within the first 2 years after publication; how about after the first 5 years? After downloading the JGR-ES data from the Web of Science, I grouped papers by year and investigated these yearly cohorts in the box and whisker plots below.
The oldest JGR-ES articles (2003-2006) recieved between 0 – 40% of their total citations in the first 2 years of their life — the JIF time window. The newest articles (2008-2010) receive a larger share of their total citations, but keep in mind that this share will continue to shrink through time as the paper accumulates more citations.
The five year fraction is larger, and seems to represent 10-80% of total citations for the oldest JGR-ES papers (2003-2006).
These plots demonstrate some of the complications with JIFs for journals where citations might accrue more slowly that a 2 and 5 year window.