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Journal Citation Analysis

The general formula for calculating an n-year (shifted) diachronous JIF
in the publication year Y is:
P s1n21
s
JDIF ðYÞ 5 i5s CITðY 1 i; YÞ (6.6)
n
PUBðYÞ
where s 5 0,1,2, .. . denotes a possible shift with respect to the publica-
tion year Y.
Citation data needed to calculate a diachronous impact factor can
always be found in the same column of a p-c matrix. The term “diachro-
nous” refers to the fact that the data needed to calculate it refer to differ-
ent years: one has to “move through time” (dia 5 through and
chronos 5 time in ancient Greek). The diachronous JIF as calculated by
formula (6.6) for a given year Y is increasing in n (more precisely, non-
decreasing) and reflects an evolution over time.
The time interval used to calculate an impact factor, be it synchronous
or diachronous is called the citation window.
One may also calculate a relative diachronous JIF by dividing
s
s
JDIF ðYÞ by n leading to: JDIF ðYÞ=n. This indicator will usually, after an
n n
initial increase, decrease over time.
The 1-year journal diachronous impact factor without a time shift,
0
JDIF 1 (Y), is called the journal immediacy index. It is one of the indica-
tors provided in the JCR.
6.6 MORE ON PUBLICATION-CITATION MATRICES AND
IMPACT FACTORS

We note that the data shown in p-c matrices do not necessarily have to
refer to a journal. Any set of articles, for instance those published by a
research group during a given publication year, can be represented in a
similar matrix.
A diachronous impact factor can be calculated for one-off publications
such as a conference proceedings or an edited book. This was done by
Rousseau (1997a) for Informetrics 87/88 and Informetrics 89/90 (Egghe &
Rousseau, 1988, 1990), the proceedings of the first and second sciento-
metrics and informetrics conference (which later became the ISSI confer-
ences). The pool for this calculation were all journals included in the
WoS (at that time), expanded with all proceedings volumes of this confer-
ence series. A 4-year citation window was used. This approach also pro-
vides an example of a case where the citation pool is expanded.