Median age of the articles that were cited in the JCR year. Half of a journal's cited articles were published more recently than the cited half-life.
The Cited Half-life measures all of the cites earned by a publication (across all cited years) during the JCR year (i.e., citing year = JCR year). The cited half-life asks, if we take all those citations and sort them by publication-year-of-cited-item—we can split that body of citations directly in half: cites to younger cited item years (more recently published); and cites to older cited item years (less recently published). If we do this, how far back in time (from the JCR year) does that neat halfway split occur? The answer is the cited half-life. If a publication’s cited half-life is 4.6, this means that half the citations it earned (where citing year is JCR year) were to items published 4.6 or fewer years ago. And half were to items published longer ago than that.
What does 4.6 years mean? Or more specifically, what does the .6 decimal mean? We are not talking about items published up through the first 60% of whatever year (i.e., approximately August 7). An explanation of the calculation below .
Note that the items being counted in the Cited Half-life calculation are not papers themselves, but rather cites to those papers. If a paper was published in year X and got 5 cites during the JCR year, then the tally for year X goes up by 5 (once for each cite) – it does not merely go up by 1 (for the paper).
All the citations earned by Journal X (citing year = JCR year) to any year are essentially lined up in order of the cited item publication year. This body of citations is then split in half. The “location” of this split is the Cited Half-Life.
It will often be the case, when this happens, that on whatever year the split falls (e.g., 2011)—that year will be represented on both sides of the split. This will result in a cited half-life with a non-zero decimal. For example, there may be 60 cites to 2011 in the younger half, and 40 cites to 2011 in the older half. In the 2015 JCR this would result in a cited half-life of 4.6 years.
The Cited Half-life provides context for what can colloquially be thought of either as the “shelf life” of items published in a publication (for how long do they continue to be cited)…and simultaneously as the “timeliness” of those items (how soon after publication do they begin earning most of the cites that they will ever earn).
A low cited half-life suggests citation activity that peaks and drops off quickly. A high cited half-life suggests citation activity that peaks and drops off more slowly (or taken another way, that only peaks after a lag). Neither of these are “good” or “bad”. Note that a journal will not receive a Cited Half-Life if it has earned fewer than 100 citations during the JCR year.