What does Hirsch index evolution explain us? A case study: Turkish
  Journal of Chemistry
Año 8, No.27, Ene – Mar. 2007                                                           
What does Hirsch index evolution explain us?
A case study: Turkish Journal of Chemistry
Metin Orbay, Orhan Karamustafaoğlu and Feda Öner
Amasya University (Turkey)
morbay@omu.edu.tr, orseka@yahoo.com, oner3@yahoo.com
Abstract
The evolution of Turkish Journal of Chemistry’s (TURK J CHEM) Hirsch index (h-index) 
over the period 1995-2005 is studied and determined in the case of the self and without 
self-citations. It is seen that the effect of Hirsch index of TURK J CHEM has a highly 
positive trend during the last five years. It proves that TURK J CHEM is improving 
itself both in quantity and quality since h-index reflects peer review, and peer review 
reflects research quality of a journal.
Key words 
Bibliographic citations, citation index, h-index, Hirsch number, Scientometric analysis, 
Bibliometry
Resumen
Presenta la evolución del índice Hirsch (h-index) de la Turkish Journal of Chemistry’s 
(TURK J CHEM) durante el período 1995-2005; el caso es estudiado y precisado tanto 
con auto-citaciones como sin auto-citaciones. El indice Hirsch permite apreciar una alta 
tendencia positiva de los indicadores de la TURK J CHEM durante los últimos cinco 
años. Se comprueba así que la TURK J CHEM va mejorando tanto en cantidad como en 
calidad de contenidos dado que el h-index refleja la revisión por pares, y la revisión por 
pares refleja la calidad de las investigaciones de una revista.
Palabras clave
Citas bibliográficas, índice de citación, indice-h, NúmeroHirsch, Cienciometría, 
Análisis cienciométrico, Bibliometría
Introduction
Hitherto, several citation-based indicators have been used to measure research 
performance (e.g. the number of citations to each of the q most cited papers, the total 
number of citations, the citations per paper, the number of highly cited published 
papers). There are valid reservations about using above mentioned indicators to 
measure performance because some papers are cited for reasons that are unrelated to 
the quality or utility of a study (see: Kelly & Jennions, 2006; Miller, 2007 and references 
therein).
essay
Orbay, Karamustafaoğlu &, Öner - What does Hirsch index evolution explain us?
Recently, taking into account above citation-based indicators with advantages and 
disadvantages, Jorge E. Hirsch has suggested a new indicator called h-index, which 
means that one single index for the assessment of the research performance of an 
individual scientist. According to the definition by Hirsch, “A scientist has index h if 
his/her N papers have at least h citations each, and the other (N-h) papers have fewer than h 
citations each” (Hirsch, 2005). Hirsch’s article has generated considerable interest and 
almost immediately provoked reactions in the scientific community (Ball, 2005; Braun, 
Glanzel & Schubert, 2005; Glanzel & Persson, 2005; Glanzel, 2006a; Egghe & Rousseau, 
2006; Egghe, 2006; Cronin & Meho, 2006; Burrell, 2007; Rousseau, 2007a). The h-index 
has generally well received by the research group. Of course, the h-index has also a 
number of disadvantages as point out by some authors (Kelly & Jennions, 2006; Van 
Raan, 2006). After all these beneficial arguments, W. Glanzel has summarized some 
pros and cons of h-index in his excellent recent paper (Glanzel, 2006b).
After a short time, the h-index definition has been adapted into journals and article 
citations, as a h-type index-equal to h if you have published h papers, each of which has 
at least h citations (Braun, Glanzel & Schubert, 2006). T. Braun and co-workers stressed 
that the h-type index for journals would advantageously supplement journal impact 
factor (IF), the total number of citations divided by the number of articles (Garfield, 
1955), at least two aspects: respectively,
i. It is robust in the sense that it is insensitive to an accidental excess of uncited 
articles, and to one or several highly cited articles,
ii. It combines the effect of “quantity” and “quality” in a rather specific.
Naturally, the journal h-index would not be calculated for a “lifetime contributions”, as 
defined by Hirsch for the scientific output of a researcher, but for a definite period-in 
the simplest case for a given year. Using this procedure, R. Rousseau studied the 
evolution of the Journal of American Society of Information Sciences’ Hirsch index and 
introduced relative h-index (Rousseau, 2007b).
In this opinion article, the evolution of Hirsch index of Turkish Journal of Chemistry 
(TURK J CHEM) over period 1995-2005 is studied and determined in the case of the self 
and elimination self-citation (or without self citation) of the journal.
Method and results
As is well known, Web of Science database offers a very simple way to determine the 
annual h-index of a journal. Retrieving all source items of a given journal from a given 
period and shorting them by the number of “times cited”, it is easy to find the h-index 
of the journal for the given year (http://isiknowledge.com).
In this study, we conduct a case study for h-index of TURK J CHEM over period 1995-
2005. Meanwhile, we consider a fixed moment in time when citations are collected 
from Web of Science (http://isiknowledge.com, retrieved date 24.12.2006). The h-index 
of TURK J CHEM over the period 1995-2005 is determined in the case of the self and 
without self-citations, as shown in Figure 1.
Año 8, No.27, Ene – Mar. 2007                                                           
h-index h-index(w ithout self citations)
Figure 1. h-index of TURK J CHEM.
However, besides the period over which a volume can collect citations, also the 
number of articles published in that volume influences the h-index. For this reason, the 
h-index must be divided by the number of articles published, leading to a normalized 
(or relative) h-index (Rousseau, 2007b). In this case, the results are shown with self and 
without self citations for the journal in Figure 2.a. and Figure 2.b., respectively.
Figure 2a. Normalized h index with self citiations of TURK J CHEM.
Orbay, Karamustafaoğlu &, Öner - What does Hirsch index evolution explain us?
Figure 2b. Normalized h index with eliminated self citations of TURK J CHEM.
As can be clearly seen from Figure 2, using the normalized h-index leads to a linear 
increase when going backward in time (or decrease when going forward in time). The 
Pearson correlation coefficients of the regression lines of this journal are 0.686 for 
normalized h-index (continuous line in Figure 2.a) with self citations and 0.689 for 
normalized h-index without self-citations (dot line in Figure 2.b), which are moderate, 
but statistically significant (1% level). It is not surprising that these two correlation 
coefficients are very close to each other because of the fact that the self-citations over 
this period are limited by approximately 20%. On the other hand, we encounter that 
this value is high in other twenty randomly selected journals published in the same 
field.
It is obvious from the Figure 1 and Figure 2 that h-index and normalized h-index are 
extremely different trend between 1995-2000 and 2000-2005 periods. So, we focus on 
two periods. In the former period, the Pearson correlation coefficients are 0.336 for 
normalized h-index with self citations and 0.192 for without self-citation, which are
very low, but statistically significant (1% level). It is not surprising that TURK J CHEM
has started to be scanned in Web of Science newly in this period. For this reason, it can 
be thought that a few researchers were aware of this journal. On the other hand, in the 
latter period, the Pearson correlation coefficients are 0.858 for normalized h-index with 
self citations and 0.941 for without self-citation, which are very high, and statistically 
significant (1% level). From these interesting results, we conclude that a lot of 
published papers in this journal have been very high impact with respect to “quantity”
(number of publications) and “quality” (citation rate), recently. Furthermore, it is 
known that TURK J CHEM has started open access in the latter period. Thus, we think 
that open access contributes h-index of this journal.
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Año 8, No.27, Ene – Mar. 2007                                                           
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