Overview of Journal Metrics

pISSN 2288-8063 Sci Ed 2018;5(1):16-20 eISSN 2288-7474 https://doi.org/10.6087/kcse.112 Review

Overview of journal metrics

Kihong Kim1, Yeonok Chung2 1Department of Energy Systems Research and Department of Physics, Ajou University, Suwon; 2Department of Social Welfare, Jangan University, Hwaseong, Korea

Abstract Various kinds of metrics used for the quantitative evaluation of scholarly journals are reviewed. The impact factor and related metrics including the immediacy index and the aggregate impact factor, which are provided by the Journal Citation Reports, are explained in detail. The Eigenfactor score and the article influence score are also reviewed. In addition, journal metrics such as CiteScore, Source Normalized Impact per Paper, SCImago Journal Rank, h-index, and g-index are discussed. Limitations and problems that these metrics have are pointed out. We should be cautious to rely on those quantitative measures too much when we evaluate journals or researchers.

Keywords Eigenfactor score; h-index; Impact factor; Journal metrics; SCImago Journal Rank; Source Normalized Impact per Paper

Introduction

There exist a variety of metrics that are used to indicate the level and the influence of scholarly journals. Most of these metrics are obtained by analyzing the citation data of journal articles. Among them, the impact factor is the best-known and most influential index. This index is calculated by a very simple and easy method, but it also has several problems. A number of other metrics have been proposed for the purpose of correcting these problems and providing more reliable estimates. In the present review, we introduce the definitions of several journal metrics and the methods to calculate them and explain their characteristics and defects briefly. Received: January 21, 2018 Accepted: February 12, 2018 Impact Factor and Related Metrics Correspondence to Kihong Kim [email protected] The idea of impact factor was proposed by Eugene Garfield in 1955 [1]. The Science Citation ORCID Index (SCI) was created based on this idea in 1964 and a quantitative evaluation of scholarly Kihong Kim journals was launched for the first time. This index is annually announced in the Journal Cita- http://orcid.org/0000-0001-9965-3535 Yeonok Chung tion Reports (JCR), which is currently managed by Clarivate Analytics, and is widely used by http://orcid.org/0000-0003-0008-5772 academic communities. Many related indices are also announced in the JCR.

This is an open access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/ licenses/by-nc/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Impact factor, 5-year impact factor, immediacy index, and becomes 50% up to the first digit after the decimal point, then impact factor without self cites we find that the cited half-life is 9.1 years. This index mea- In a given year, the impact factor of a certain journal is de- sures for how long the published contents are cited. In a simi- fined as the average value of citations per paper received by lar manner, one can calculate the citing half-life using the pa- the items published in the journal in two previous years. More pers cited by the journal J. specifically, its definition is given by Impact factor of the journal J in the year X= A/B, Median impact factor and aggregate impact factor where A is the number of total citations in the year X received There is a problem with the impact factor in that it shows by all items published in the journal J in the years (X-1) and rather large variations among academic disciplines. For that (X-2) and B is the total number of all citable items published reason, the JCR classifies journals based on the subject cate- in the journal J in the years (X-1) and (X-2). Citable items in- gory and provides several metrics representing each category. clude only papers and reviews and do not include errata, edi- The median impact factor is that of the journal placed pre- torials and abstracts. In the counting of A, however, citations cisely in the middle when the journals in a certain category to all items published in J are included. are arranged in the order of their impact factors. When the The 5-year impact factor in the year X is similar to the or- total number of journals in the category, N, is an odd number, dinary (2-year) impact factor, except that it is calculated using it is the impact factor of the [1+(N-1)/2]-th journal. When N the citation data during the 5 years from the year (X-1) to the is even, it is the average of the impact factors of the (N/2)-th year (X-5). This index is useful in the academic disciplines and [1+N/2]-th journals. where the number of citations is small or it takes some time The aggregate impact factor is obtained by dividing the to- for published results to be accepted by many researchers. On tal number of citations received by all items published in all the other hand, the immediacy index is calculated similarly to journals in a certain category in the year X by the total num- the impact factor using the total number of citations received ber of citable items published in all journals in that category in the year X by all items published in the same year X. If this in the years (X-1) and (X-2). Since the distribution of impact index is large, it means that the papers published in that jour- factors is not linear but highly skewed, the aggregate impact nal are cited rather quickly. factor tends to be substantially larger than the median impact The journal self-citation means the case where a paper factor, as can be seen in Table 2. The aggregate immediacy in- published in the journal J is cited in the same journal. In the dex, the aggregate cited half-life, and the aggregate citing half- JCR, the impact factor without self cites, which is obtained af- life are also provided in the JCR. ter excluding journal self-citations, is also announced. If the difference between the impact factor and the impact factor Problems of the impact factor and the editorial ethics without self cites is significantly large for a certain journal, As we mentioned already, there is a problem with the impact sometimes that journal is excluded from the JCR list. factor in that it shows large variations among academic disciplines. In Table 2, we show the aggregate impact factor, the Cited half-life and citing half-life median impact factor, the aggregate cited half-life, and the av- The cited half-life is calculated using the number of citations erage number of citations per paper for several subject catego- received in the year X by all items published in a certain jour- ries listed in the JCR in 2011 and 2013. We notice a trend that nal in all years. For example, let us suppose that the journal J the impact factors are usually larger in the disciplines where received 1,285 citations in 2017. In Table 1, we show the (hy- more papers are cited on average and the cited half-life is shorter. pothetical) number of citations and the cumulative percent- The impact factor is obtained by the arithmetic mean of age classified by the published year of cited items. We find the number of citations received by the items published in a that the cumulative percentage becomes 50% between 2009 certain journal. However, it is well-known that the distribu- and 2008. If we assume that papers were cited equally in every tion of the number of citations in a given journal is highly month and calculate the year when the cumulative percentage skewed. There exists a tendency that the impact factor overes-

Table 1. Number of citations received in 2017 and its cumulative percentage classified in terms of the published year of cited items

2017 2016 2015 2014 2013 2012 2011 2010 2009 2008 2007–all

Citations in 2017 23 65 147 138 58 44 51 45 68 62 584 Cumulative percentage 1.79 6.85 18.29 29.03 33.54 36.97 40.93 44.44 49.73 54.55 100

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Table 2. The aggregate impact factor, the median factor, the aggregate cited half-life, and the average number of citations per paper for several subject catego- ries listed in the Journal Citation Reports in 2011 and 2013 Average number of Aggregate impact factor Median impact factor Aggregate cited half-life Subject category citations per paper 2011 2013 2011 2013 2011 2013 2011 2013

Cell biology 5.760 5.816 3.263 3.333 6.9 7.2 53.4 55.0 Chemistry, multidisciplinary 4.738 5.222 1.316 1.401 5.9 5.6 40.9 44.6 was proposed by Brin and Page [3] and has been used in the Google search engine. In order to calculate the Nanoscience & nanotechnology 4.698 was4.902 proposed by Brin1.918 and Page1.768 [3] and has been3.8 used in the4.1 Google search35.5 engine. In39.1 order to calculate the Astronomy & astrophysics 4.242 Eigenfactor4.462 score,1.683 we first define1.676 a database consisting6.8 of 7.0N journals and49.3 construct an53.2 matrix H, the ij Materials science, multidisciplinary 3.107 3.535 1.132 1.380 5.2 5.4 32.2 34.8 component of which is given by �� Physics, multidisciplinary 2.680 component2.953 of which0.983 is given by1.300 7.7 8.0 30.4 33.8 Engineering, mechanical 1.232 1.573 0.743 0.889 7.6 8.0 25.2 28.1

Mathematics 0.709 0.729 0.561 0.582 > 10.0 > 10.0 �� 19.8 21.0 �� , � � ∑�� , where represents the number of citations in the journal∑ j in� the year X received by the items published in the timates the importance of individual papers. In other�� words, journal self-citations are excluded in the calculation of the Ei- journal �i ��during the five years from the year (X-5) to the year (X-1). Since journal self-citations are excluded in the most papers are cited substantially less thanjournal what i duringthe journal the five yearsgenfactor from the score, year all (X-5) diagonal to the elementsyear (X-1). of Sincethe matrix journal Z self-citationsare zero. are excluded in the impact factor indicates. Therefore it is notcalculation accurate of to the judge Eigenfactor Next score, we define all diagonal a vector, elements a, called of the the matrix article Z arevector. zero. The Next i-th we define a vector, a, called the quality of an individual paper or researcher based on the component of this vector, ai, is obtained by dividing the total journal impact factor. the article vector. The i-th numbercomponent of papersof this vector,published , inis obtainedthe journal by dividingi during thethe total5 years number of papers published As competition among scholarly journals becomes stron- from the year (X-5) to the year (X-1) by the total number of �� ger, it sometimes occurs that some journalin editors the journal adopt i during poli- the papers5 years published from the year in the (X-5) whole� to th databasee year (X-1) during by the the total same nu perimber- of papers published in cies to manipulate journal impact factor deliberately. One od. In the calculation of this kind of problem, one needs to the whole database during the same period. In the calculation of this kind of problem, one needs to take a special practice that is ethically troubling is to induce authors to do take a special care of the dangling nodes and the dangling clus- journal self-citation. Publishing more reviewcare of papers the dangling than is nodes ters. and Anthe danglingexample clusters.of the dangling An example node of is the the dangling case where node a iscer the- case where a certain necessary and publishing papers which have a higher chance tain journal j does not cite any of the journals in the database, of citation deliberately at the beginning ofjournal a year j doesare similarnot cite any butof the its journalspapers are in thecited database, by other but journals. its papers Then are cited the matrix by other ele journals.- Then the matrix practices. This behavior occurs because too much importance ments Zkj are zero for all k. Since the j-th column of the matrix elements are zero for all k. Since the j-th column of the matrix H is undefined, it is necessary to replace this is given to the impact factor and distorts theelements metric unfairly. are zero forH all is k. undefined, Since the j-thit is column necessary of the to replacematrix H this is undefined,column by it a is suit necessary- to replace this �� able vector. We define a matrix H*, which is obtained by re- column by� a suitable vector. We define a matrix , which is obtained by replacing all columns corresponding to the placing all columns corresponding to the dangling nodes by Eigenfactor Score and Article Influence Score ∗ � dangling nodes by the articlethe vectorarticle avector, and then a, and introduce� then introduce an N×N matrixan N× P N given matrix by P giv- The Eigenfactor score and the journal influence score were en by developed by Bergstrom et al. [2] to overcome the defects of � � the impact factor and have been provided by the JCR since ∗ � �� ∗ ⋮⋱⋮ 2007. The concept of Eigenfactor is based on the theory of � � α� � �� ⋮⋱⋮�, �� complex networks. For its calculation, onewhere uses a method is an appropriate sim- where constant α is a ndan is appropriate usually selected constant to be� and0.85.�� is The usually journal selected influence to vector, v, is defined to ilar to the PageRank algorithm, which was proposed by Brin be 0.85. The journal influence vector, v, is defined to be the was proposed by Brin and Page [3] and has been used in the Google search engine.α In order to calculate the and Page [3] and has been used in the Googlebe the searcheigenvectorα engine. corresponding eigenvector to the corresponding largest eigenvalue to the of thelargest matrix eigenvalue P. The i- thof componentthe ma- of the vector v has the Eigenfactor score, we first define a database consisting of N journals and construct an matrix H, the ij In order to calculate the Eigenfactor score,meaning we first of thedefine weighting a trixfactor P. represenThe i-thting component the relative of importancethe vector vof has the thejour meaningnal i in the of group of journals in the database consisting of N journals and constructmeaning an of N× the N weighting ma- thefactor weighting represen tingfactor the representingrelative importance the relative of the jourimportancenal i in the of group of journals in the component of which is given by �� trix H, the ij component of which is given database.by Finally, the Eigenfactthe journalor score i ofin thethe journal group i,of journals, is calculated in the usingdatabase. Finally, the Eigenfactor score of the journal i, Fi, is calculated using � �� , ∑�� where Zij represents the number∑ of� citations in the journal j in � �∑ �� where represents the number of citations in the journal j in the year X received by the items published in the � � �� . the year X received by the items published Accordingin the journal to this i dur definition,- According the sum ofto allthis Eigenfactor definition, ∑∑scor the��es sum ��for �allof�� journalsall�� Eigenfactor in the database scores is equal to 100. Since �� According to this definition, the sum of all Eigenfactor scores for all journals in the database is equal to 100. Since journal �i during the five yearsing thefrom five the years year (X-5)from tothe the year year (X-5) (X-1). to Since the year journal (X-1). self-citations Since arefor excludedall journals in thein the database is equal to 100. Since this quan- this quantity is not normalized by the total number of papers published in a given journal, it tends to be larger for calculation of the Eigenfactor score, all diagonal elements of the matrix Z are zero. Next we define a vector, a, called journals publishing larger number of papers, if all other conditions are the same. A useful characteristic of the the article vector. The i-th component of this vector, , is obtained by dividing the total number of papers published 18 | Sci Ed 2018;5(1):16-20 Eigenfactor is that it makes it possible to compare journals belonginghttp://www.escienceediting.org to different academic disciplines directly � in the journal i during the 5 years from the year (X-5)� to the year (X-1) by the total number of papers published in because those differences are adjusted for in this metric. The article influence score measures the influence of the whole database during the same period. In the calculation of this kind of problem, one needs to take a special � �� care of the dangling nodes and the dangling clusters. An example of the dangling node is the case where a certain journal j does not cite any of the journals in the database, but its papers are cited by other journals. Then the matrix elements are zero for all k. Since the j-th column of the matrix H is undefined, it is necessary to replace this

�� column by� a suitable vector. We define a matrix , which is obtained by replacing all columns corresponding to the ∗ dangling nodes by the article vector a, and then introduce� an N×N matrix P given by

� � ∗ � �� α� � �� ⋮⋱⋮�, � � where is an appropriate constant and is usually selected to be� 0.85.�� The journal influence vector, v, is defined to be the eigenvectorα corresponding to the largest eigenvalue of the matrix P. The i-th component of the vector v has the meaning of the weighting factor representing the relative importance of the journal i in the group of journals in the database. Finally, the Eigenfactor score of the journal i, , is calculated using

��

� ∑�� . According to this definition, the sum of all Eigenfactor ∑∑scores for �all journals� in the database is equal to 100. Since this quantity is not normalized by the total number of papers published in a given journal, it tends to be larger for journals publishing larger number of papers, if all other conditions are the same. A useful characteristic of the

Eigenfactor is that it makes it possible to compare journals belonging to different academic disciplines directly because those differences are adjusted for in this metric. The article influence score measures the influence of

�� Overview of journal metrics

tity is not normalized by the total number of papers published pers published in a certain journal in the three previous years, in a given journal, it tends to be larger for journals publishing (X-1), (X-2), and (X-3). Among these references, we consider larger number of papers, if all other conditions are the same. only the references published during the same 3-year period. A useful characteristic of the Eigenfactor is that it makes3-year it period.The DCPThe DCP is obtained is obtained by by dividing dividing the the total total number of of those those refere nces by the number of citing papers. possible to compare journals belonging to different academicIn this calculation,references only by thecitati numberons of the of journalsciting papers. belonging In this to the calculation, database are included and other journals are 3-year period. The DCP is obtained by dividing the total number of those references by the number of citing papers. disciplines 3-yeardirectly period. because The those DCP isdifferences obtained by are dividing adjusted the for total numberonly citations of those ofrefere thences journals by the belonging number of to citing the database papers. are in- in this metric. The article influence score Ii measures theignored. in- Thecluded RDCP and is other obtained journals by normalizing are ignored. the TheDCP RDCP by the ismedian obtained DCP of the database. In this calculation, only citations of the journals belonging to the database are included and other journals are individual papers publishedfluence in the of journal individualIn this i andcalculation, ispapers defined publishedonly by citati ons in theof the journal journals i and belonging is byto thenormalizing database are the included DCP by and the other median journals DCP are of the database. defined by ignored. The RDCP is obtained by normalizing the DCP by the median DCP of the database. ignored. The RDCP is obtained by normalizing the DCP by the median DCP of the database. SJR SJR �� The SJR is provided by the Scopus together with the SNIP [5]. �� .�� . This quantity can be used as an alternative�� to the impact Thefac- SJR isIt providedis calculated by the iteratively Scopus together in the followingwith the SNIP manner. [5]. It First,is calculated one iteratively in the following manner. This quantity can be used as an alternative to the impact factor. The meanSJR article in the entire JCR database has an tor. The meanSJR article in the entire JCR database has an article introduces a vector S, which is meant to represent the relative First, one introduces a vector S, which is meant to represent the relative importance of the journals belonging to the article influence of 1. influence of 1. The SJR is provided by theimportance Scopus together of the with journals the SNIP belonging [5]. It is calculated to the database iteratively of in N the following manner. The SJR is provided by the Scopus together with the SNIP journals.[5]. It is calculated Si is the weighting iteratively factorin the followingof the journal manner. i. In the first database of N journals. is the weighting factor of the journal i. In the first stage of the iteration, the values of CiteScore, Source Normalized ImpactFirst, perone introducesPaper, a vectorstage S, whichof the is iteration, meant to therepresen valuest the of relative Si are assigned importance arbitrarily. of the journals belonging to the First, one introduces a vector S, which is meant to represent the relative importance� of the journals belonging to the � and SCImago Journal Rank are assignedThe arbitrarily. final result The� does final not resu dependlt does not on depend the choice on the of choice the initial of the initial values. In the next step, the � database of N journals. is the weighting factor of the journal i. In the first stage of the iteration, the values of database of N journals. is the weighting factor of the journalvalues. i. In the firstnext stage step, of the the updated iteration, values the values of Si areof calculated In this section, we review three journal metrics providedupdated by �valuesusing ofthe formula are calculated using the formula � CiteScore, Source Normalized Impact per Paper, and� SCImagoare assigned Journal arbitrarily. Rank The� final result does not depend on the choice of the initial� values. In the next step, the � the Scopus aredatabase, assigned which arbitrarily. are the The� CiteScore, final resu thelt does Source not Nordepend- on the choice of the initial values. In the next step, the � �� In this section, we reviewmalized three jo urnalImpact metrics per Paperprovided (SNIP), by the andScopus theupdated dataSCImagobase, values which Journal of are arethe calculatedCiteScore, using the the formula Rank (SJR).updated values of are calculated using the formula � ∗ � �� ∑�� Source Normalized Impact per Paper (SNIP), and the SCImago� Journal Rank (SJR). � � � � ∗ � � � � � � e� �� , �� dangling nodes� ��dangling nodes� CiteScore � � ∗ ∑∑� � where the constants d and e are chosen to be d = 0.85 and e = 0.1 and the matrix and the article vector a are � ∗�� ∑�� The CiteScore is very similar to the impact factor.�� It �is calcu- � � � ∗ � � � ∗ � �� ∑ � � � e� �� , � � � ∗ �� dangling nodes� � � ��dangling nodes� CiteScore lated using the Scopus data and� ise� defined�� as the average�� value�defined�� similarly �∑∑ to the � � case� �� of� the Eigenfactor�� calculation, � � , except that the 3-year �citation window is used. Using the � ∑∑�� dangling nodes� ��dangling nodes� of citations per item received by the itemswhere published the constants in the d and ewhere are chosen the constants to be d = d 0.85 and ande are e chosen= 0.1 and to thebe dmatrix= 0.85 and ande= 0.1the article vector a are where the constants d and e are chosen to be d = 0.85 and e = 0.1 and the matrix and the article vector a are ∗ The CiteScore is very similar to the impact factor. It is calculated using the Scopus dataupdated and is definedvalues of as the, newaverage calculations* are repeated until all values converge. Finally, the SJR of the journal i is journal in three previous years, rather thandefined in twosimilarly previous to the caseand of thethe matrixEigenfactor H and calculation, ∗the article except vector that a theare 3-yeardefined �citation similar window- is used. Using the � years as in definedthe case similarly of the impactto the case factor. of the Another Eigenfactor difference calculation, ly except to the that case� the of 3-year the Eigenfactorcitation window calculation, is used. Using except the that the value of citations per item received by the items published in the journal in three previouscalculated years, ratherusing than� in two from the impact factor is that both numeratorupdated and values denomina of ,- new 3-yearcalculations citation are window repeated isuntil used. all Usingvalues theconverge. updated Finally, values the of SJRSi, of the journal i is tor include updatedall document values types.of , new calculations are repeated until allnew values calculations converge. are Finally, repeated the SJR until of allthe values journal converge. i is Final- previous years as in the case of the impact factor. Another difference from the impact factor�� is that both numerator � calculated using ly, the SJR of the journal i is calculated using calculated using � � and denominator include SNIPall document types. � �� , The SNIP was proposed by Moed [4] as a metric that adjustswhere is the total number of papers published in the journal� i during the 3-year period. � for different citation patterns across different academic disci- � � � � � �� , plines. This metric is provided in the Scopus and can be used where Ai is the total number of �papers published in the jour- SNIP where is the total�� number � of�, papers published in the journal i during the 3-year period. instead of thewhere impact isfactor. the total The number SNIP is of defined papers publishedas in the journalnal� i duringi during the the 3-year 3-year period.period. � � � The SNIP was proposed by Moed [4] as a metric� SNIP= that adjust RIP/RDCP,s for different citation patterns across different academic h-index,h-index, g-index, g-index, and i10-index and i10-index disciplines. This metric iswhere provided the in acronyms the Scopus RIP and andcan beRDCP used insteadstand forof the “raw impact impact factor. The SNIP is defined per paper” and “relative database citation potential” respecThe- h-indexThe was h-index proposed was by proposed Hirsch in 2005by Hirsch [6] as ain new 2005 metric [6] foras aevalua new ting the ability of an individual as h-index, g-index, and i10-index tively. The RIPh-index, is the numberg-index, of andcitations i10-index in the year X received metric for evaluating the ability of an individual researcher. researcher. This index is calculated using all citations received by the papers published by a specific researcher. If we by the papers publishedSNIP in the= RIP three / RDCP, previous The h-indexyears, (X-1), was proposed (X- Thisby Hirsch index in is 2005 calculated [6] as ausing new metricall citations for evalua receivedting the by ability the pa of- an individual 2), and (X-3)The in h-index a certain was journal proposed divided by Hirsch by the in total2005 number[6] as a new metricpers published for evalua byting a specificthe ability researcher. of an individual If we arrange those pa- arrange those papers in the order of citations received by them and if h papers are cited at least h times, then the where the acronyms RIP andof papers. RDCP standIt is similar for “raw to impact the impact per paper” factor, researcher.and except “relative that This da the indextabase 3- is citation calculatedpers potential”in the usi orderng all citationsof citations received received by the by papersthem andpubl ifished h papers by a specific researcher. If we researcher. This index is calculated using all citations received by the papers published by a specific researcher. If we year citation window is used and only citations of papersmaximum are are number cited ofat hleast is the h h-indextimes, then of that the researcher. maximum Si ncenumber it is possible of h is to assign an h-index to the group of respectively. The RIP is the number of citations in the year X receivedarrange by the thosepapers papers published in the in order the three of c itationsprevious received by them and if h papers are cited at least h times, then the included andarrange those those of errata papers and in the editorials order of careitations excluded. received In by themthe h-indexand if h papersof that are researcher. cited at least Since h times, it is possible then the to assign an order to define the RDCP, one needs to define the DCP, whichpapers publishedh-index in to a the specific group journal of papers in a specificpublished year, in it a canspecific be used journal also as a journal metric. years, (X-1), (X-2), and (X-3) in a certain journal divided by the totalmaximum number of number papers. ofIt ish issimilar the h-index to the impactof that researcher. Since it is possible to assign an h-index to the group of means the maximumdatabase citationnumber of potential, h is the h-index first. Let of thatus considerresearcher. Sincein a it specific is possible year, to it assign can be an used h-index also to as the a journal group of metric. Since the h-index is obtained by using the total number of citations of each paper, it increases monotonically factor, except that the 3-yearthe citationreferences window of the is papersused and which only citationscited inpapers theof papers year published X are the included in pa a- specific and Since thosejournal ofthe inerrata h-index a specific is year,obtained it can by be usingused also the astotal a journal number metric. of papers published in a specific journal in a specific year, it can be used also as a journal metric. and editorials are excluded. In order to define the RDCP, one needs to define theSince DCP the first. h-index Let usis obtainedconsider bythe using the total number of citations of each paper, it increases monotonically Since the h-index is obtained by using the total number of citations of each paper, it increases monotonically references of the papers which cited in the year X the papers published in a certain journal in the three previous http://www.escienceediting.org Sci Ed 2018;5(1):16-20 | 19 years, (X-1), (X-2), and (X-3). Among these references, we consider only the references published during the same Kihong Kim et al.

citations of each paper, it increases monotonically with time. Conflict of Interest It has a shortcoming that researchers with a small number of very influential papers have low indices. In order to correct No potential conflict of interest relevant to this article was re- this shortcoming, Leo Egghe proposed a modified index named ported. g-index. This index is defined as the maximum value of g when g papers among a certain group of papers were cited at least g2 References times. The g-index is always larger than the h-index. In addition to the h-index, Google Scholar provides a metric named 1. Garfield E. Citation indexes for science: a new dimension i10-index, which is the total number of papers authored by a in documentation through association of ideas. Science certain researcher cited at least 10 times. 1955;122:108-11. https://doi.org/10.1126/science.122.3159.108 2. Bergstrom CT, West JD, Wiseman MA. The Eigenfactor Conclusion metrics. J Neurosci 2008;28:11433-4. https://doi.org/10.1523/ JNEUROSCI.0003-08.2008 In this review, we have surveyed the definitions and the char- 3. Brin S, Page L. The anatomy of a large-scale hypertextual acteristics of various kinds of metrics used for the quantitative Web search engine. Comput Netw ISDN Syst 1998;30:107- evaluation of scholarly journals. All of these metrics are ob- 17. https://doi.org/10.1016/S0169-7552(98)00110-X tained from the analysis of citation data. In addition to the 4. Moed HF. Measuring contextual citation impact of scien- metrics surveyed here, new kinds of metrics continue to be tific journals. J Informetr 2010;4:265-77. https://doi.org/ devised. More recently, interest in alternative metrics, or ‘alt- 10.1016/j.joi.2010.01.002 metrics,’ which go beyond conventional citation analysis, has 5. Gonzalez-Pereira B, Guerrero-Bote VP, Moya-Anegon F. been growing rapidly. We emphasize, however, that no metric A new approach to the metric of journals’ scientific pres- is perfect and all metrics have limits and problems. Therefore tige: the SJR indicator. J Informetr 2010;4:379-91. https:// it is necessary not to rely on quantitative measures too much doi.org/10.1016/j.joi.2010.03.002 when we evaluate journals, papers, researchers, and institu- 6. Hirsch JE. An index to quantify an individual’s scientific tions. research output. PNAS 2005;102:16569-72. https://doi. org/10.1073/pnas.0507655102

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