191

IMPACT OF STUDIES PUBLISHED IN THE INTERNATIONAL LITERATURE BY SCIENTISTS AT THE

NATIONAL UNIVERSITY OF MEXICO

Héctor DELGADO and Jane M. RUSSELL, Centro de Informaci6n Cientifica y Humanistica,

Universidad Nacional Aut6noma de México, Ciudad Universitaria, 045 10 México, DF, México

ABSTRACT

A total of 2192 articles published in the international literahre with UNAM (National University of Mexico ) first author affiliation and registered by the CICH (Centro de Informacih Cientifica y Humanistica) BIBLAT database from 1978 - mid-1987 were included in Our analysis. Distribution of articles according to the main subject areas of the 692 different journal titles used was as follows: Physics 24.1%, Medicine 19.7%, Biology 19.4%, Chemistry 9.7%, Engineering 8.9%, Exact Sciences 7.3%, Geosciences 4.7%, Psychology 0.96%, Agrosciences 0.27%. Thirty-seven percent of articles were published in journals with a known impact factor for 1987 of 51, 46.1% (920) in journals within the range of >1-3 average citationdarticle and only 16.4% (327) in those titles with a factor >3. Fifty-four percent (1082) of studies appeared in journals whose total citation count for 1987 was 55000; 7.3% (146) in journals cited >50,000 times in that same year. UNAM scientists therefore as a group tend to publish in journals whose articles are not frequently cited in subsequent publications thus limiting their impact and visibility in the international scientific literature.

RESUME

Un ensemble de 2192 articles publié dans la littérature internationale, indexés dans la base de données BIBLATdu CICH (Centro de Informacion Cientifica y Humanistica) de 1978 à mi-1987, avec I’UNAIM (Université Nationale du Mexique) comme affiliation du premier auteur ont servi de référence à notre analyse. La répartition de ces articles selon les principaux domaines des 692 différents journaux concernés est comme suit: Physique 24.1 %, Médecine 19.7%, Biologie 19.4%, Chimie 9.796, Sciences de l’lngénieur 8.996, Sciences Exactes 7.3%, Sciences de la Terre 4.796, Psychologie 0.9696, Sciences Agricoles 0.27%. Trente sept pourcent des articles sont publiés dans des journaux ayant un facteur d’impact pour 198751, 46,196 (920) dans des journaux recevant entre 1 et 3 citations en moyenne par article et seulement 16.4% (327) dans des journaux ayant un facteur >3. Cinquante quatre pourcent (1082) des &tudes ont été publiées dans des journaux dont le nombre total de citations au cours de l’année 1987 était 6000; 7,3% dans des journaux cités plus de 50,000 fois au cours de la même année. Les chercheurs de 1’WUNAMont en effet tendance àpublier dans des joumaux dont les articles sont peu cités, limitant ainsi leur impact et leur visibilité dans la littérature scientifique internationale.

192 Hector DELGADO and Jane M. RUSSEL

Much attention has been focussed over the last decade on the accountability of science. Ever tightening research bu overnments and other funding bodies to take a critical look projeets they sponsor and to assess the relative performances of different institutions and research groups when assigning resources (1).

Criticisrm aimed at the peer review process, such as partihality of peers, lack of uniformity of criteria and high administrative costs, have forced sponsoring agencies to look for supplementary ways of evaluating resarch (2). In the search for a quick, in-house aid 'to traditional assessment procedures for quantitative 1: decision-making, the application of reiable bibliometric indiators for evaluating research output h a been the subject of different studies (3 ,4 ,5 ,6) . Althsugh the application of biblismetric amd other quantitative criteria hm been shhswn to be useful in guiding pslicy making dwisions, it is generally agreed that these have to be applied with caution and it is unlikely that they can be considered as a possible substitute for the traditional peer review procedure. Rather the intention is for quantitative analysis to provide data for the formulation ofpersonal judgements and sometimes even for it to challenge received wisdom (7).

It is important to bear in rmind that most of the discussions and methsdologiieal developments of science indicaton have been mrried out within the conceptual framework of the Fint W countries (8). How then is their role to evaluate Third World research env ? Unlike studies from csuntries at the centre of world scientific activity, rch from the periphery is disseminated mainly through national publications, in a variety of document formats and predominsntly in the native lanpages of the esuntries eomcerned (9 , lO) . In consequeance, only a smalll percentage of Third World smdies rexh the important I

international bibliograp'hic services which makes thhem generally available for bibliometric monitoring or screeming. This situation is especially eritical with respect to the small number of documents from developin countries thhat are processed by the Science Citation Index, taking into consideration that this is the main and, certainly in the case of the developing world, the most commonly used tool for mrrying out citation studies (1 1 ). However, reports from the pekpheral esuntries which do reach the imtemational literature have the advanntage of publication in a widely understood language (predominantly English) and in those journals which are generally available worldwide for consultation, evaluation and subsequent citation. Perhaps more importantly, publication in core joumals assures an acceptable international standard of presentation and level of science (12). Therefore, assessing what a Third World country or sector of this country publishes in the international literature will give us a clwr idea of their researeh which is reaching a wide audience. This then represents their science which is internationally most visible. Also by evaluating production in the international literature, results at both macro (region, country) and micro

Impact of International Publications by Mexican Scientists 193

(institute, research group or individual researcher) levels for Third World research are, at least in theory, capable of comparison quality-Wise with their First World equivalents.

In order to get a general picture of the research published by Our institution (National University of Mexico, UNAM) which reaches an wide audience, we have carried out a study of the articles published in the international literature by Our researchers in different disciplines over a ten year period. We identified areas of high visibility and determined the potential impact of UNAM research based on an evaluation of the impact factors and total citation counts of the joumals where the studies were published. Although the use of impact factors to measure the relative quality of scientific journals has come under scrutiny (13), analysis of the citation patterns of journals will give us a valid indication of the visibility of the articles they publish and inform us about the publication strategies of the authors in question (14). When examining the research activity of a large group of researchers, as in Our case, the analysis of journal impact factors and citation counts gives a ready, if somewhat rough method of evaluating publication patterns, taking into consideration that publication in high impact and highly cited journals will give the studies the potential or maximum opportunity to achieve high citation rates. In general, quality of research from Third World countries is equated with publication in international journals, although scientists from developing countries take into consideration other factors, such as the applicability of results at local level, when deciding whether to publish research results nationally or internationally (15).

RESEARCH LN THE UNAM

The National Autonomous University of Mexico is the largest institution for higher education in the country and one of the oldest and most prestigious in Latin America - its origins go back to the sixteenth century. Research at the UNAM is currently organised into two separate areas: the humanities and the sciences. The former has more than 800 specialists working in 16 different institutes and centres. Research in science and engineering takes place in 15 institutions, eight centres and three University programmes where more than 1,700 scientists, scholars and technical staff work. Research is also carried out in the schools of Science, Engineering, Chemistry, Medicine and Veterinary Medicine, as well as in other schools located on five campuses off the main University City campus at the southern end of Mexico City. Besides the on- campus activity, scientific and technological research takes place in the University's field stations, observatories, ocean-going vessels, laboratories and national service centres located in different regions of the Mexican Republic.

Basic research is carried out which contributes both to universal scientific knowledge and to the solution of the country's divetse problems. Of high priority is research relating to health, energy and food. Figure 1 shows the number of

194 Hector DELGADO and Jane M. RUSSEL

searchers (excluding research technicians and postgmduate acholaa) the area of science and technology durin the y a r s comprising our e figures do not include researchers in

working in faculties or schools. However, the core of repmsented by these data.

The 1978 - mid- 1987 records of the BlIBLAT database produced by the Science and Humanities Information Centre (Centro de Infomacibn Cientifica y Humanistica, CICH) were searched for documents with UN affiliation. BIB coven papers published in the international literature indexe$ both by Science Citation Index (SC%) md Social Sciences Citation Index (SSCI). Individual journal titles used were identified and manually classified into subject categories using as far as was possible the section of Journals Wanked by Chtegoy - Ranked by Impact Factor of the 1987 (Vol. 19) SC1 Journal Citation We~orts. Only one subject eategory was assigned to each journal title. These categories were then assigned to one of the following ten major disciplines: Agrosciences, Biology, Chemistry, Engineering, Exact Sciences, Geosciences, Medicine, Physics, Psychology, Social Sciences and Humanities. Journak of a multidisciplinary nature were classified as such. 'Vlrhere titles did not appear in this list we assigned subject categories to them after consulting well-known periodials directories, such as that published by Ulrich. The impact factors for 1987 and the total number of citations received by each journal title during 1987 irrespective of the years cited (al1 yeaa column) were identified using the 1987 (vol. 19) SC1 Journal Citation Wemrts (Journal Wankings Section 1 SC9 Journals in Alphabetieal Order). Articles were assigned the impact factors and total citation counts o rnal titles where they were putplished. The impact indicator we used for articles is comparable with Braun and his group's

cted citation rate for articles (14).

total of 2192 articles were retrieved with UN first author affiliation which had been introdueed into the BIBLAT database from 1978 ts mid-1987. 'These were published in 692 different journal titles. Figure 2 relates the number of uNBR/f articles published yearly to the total number of articles with Mexico first author affiliation in the BIBLAT database. The percentage UNAM contribution to total Mexican production in the international litemture over these years is of the order of 40%. A gradua1 rise in both UNAM production of articles and for Mexico as a whole was noted frsm 1978 to 1985.

Impact of International Publications by Mexican Scientists 195

Almost one quarter of the articles authored by the UNAM were published in journals in the field of physics, followed by around 20% in both the medical and biological fields, and a further 10% in chemistry. Less than 3% of articles related to the humanities and social sciences (Figure 3). The differences in publishing patterns between disciplines is apparent in this figure. Note that in the physics field, 24% of total articles was published in just 13% of the 692 journal titles whereas in engineering a similar percentage of journal titles was used to publish only 8.9% of articles. This indicates that the physicists concentrated their articles in a reduced number of journals, whereas the engineers showed a dispersion of articles publishing fewer studies in the same number of journal titles. Figures 4 and 5 illustrate this point more clearly with the physicists publishing a large number of articles (up to 23) in certain journal titles. When examining al1 fields as a whole we found that half the titles identified were used only once. At the other extreme we found individual journals where up to 64 UNAM first author articles had been published during the ten year period (Figure 6).

A total of 117 (16.9%) journal titles, 39 of which were in the field of social sciences and humanities, had unknown 1987 impact factors. The corresponding figure for articles was 196 (8.9%). The impact factors for the remaining journals and their corresponding articles are shown in Figure 7. A total of 37.5% (749) of articles were published in journals with an impact factor of 5 1,46.1% (920) with impact factors >1 and 53, and only 16.4% (327) in journals with a factor >3. Two articles were published in the medical journal, The Lancet, which has an impact factor of 13.25, three and eight respectively in the mutltidisciplinary journals, Science with a factor of 14.30 and Nature with a factor of 14.99, and one in Review of Modern Phvsics with a high impact factor of 16.28 characteristic of review journals. Fourteen articles were published in the Journal of Neuroscience Research with the highest impact factor found in this study of 22.27. Figure 8 gives a breakdown of journals and articles with impact factors of 51. The majority of these articles (60%) were published in joumals with impact factors of > O S .

Table 1 gives a breakdown of impact factors of journals and articles published in the areas of physics, engineering and medicine. In physics 39.8% of articles had impact factors 51, in engineering, 74.4% (100% had impact factors 52) and in medicine, 30.1%. The distribution of articles in journals with impact factors 51 .O showed around 55% with figures of 50.5 in physics and in medicine, while in engineering approximately 40% were in this range.

Figure 9 shows the distribution of the total citation counts given to each journal title during 1987. One hundred and thirteen titles (16.3%) and their corresponding 190 (8.7%) articles were eliminated from this part of the study because of absence of data. Over 76% of journals and 54% of articles had total numbers of citations of 55000. The breakdown of documents with total citation counts of 55000 is illustrated in Figure 10. Approximately 68% of articles were published in journals which received 2000 citations or less during 1987.

196 Hector DELGADO and Jane M. RUSSEL

Table 1. Impact sf'joumal titles in physics, engineering and medicine

Table 2. Total number of citations given ts journal titles in physics, engineering and medicine

Impact of International Publications by Mexican Scientists 1 97

Table 2 analyses the distribution of total citation counts in three different disciplines. The percentage of articles published in journals with total numbers of citations 55000 were 40.6% in physics, 92.3% in engineering and 56.7% in medicine. Over 16% of physics articles were published in journals which received between 60,000 and 90,000 citations. When examining articles published in journals with a total citation count 55000, percentages of articles and journals in al1 three disciplines were heavily biased towards citation counts of 52000.

DISCUSSION

According to the figure of 3335 total 1981-1985 production of papers in SC1 with first author Mexican affiliation (14) and based on Our total of 1459 UNAM papers in the same period, the extent of UNAM participation in Mexican research published in the international literature is around 43%. This figure is in keeping with Our calculation of 40% using the BTBLAT database. However, this figure does not take into account articles in the international literature where the UNAM and other Mexican scientists appear as coauthors. Nor does it consider publications in national journals and other locally edited document formats. The institutes and centres for scientific research in the UNAM edit 12 journals which are heavily used by UNAM researchers to publish their results.

When we equate the number of articles published yearly in the international literature by UNAM first authors with the number of core UNAM scientists, we find an increase from 0.24 articles per researcher in 1979 to 0.38 in 1985. This suggests consolidation of research groups reflected in higher productivity in the international journals. Nevertheless, the research output per scientist is still low and suggests an average production of only one article every 2 or 3 years. However, with respect to other scientific communities in countries, such as Spain and Venezuela, which like Mexico have no strong tradition of scientific research, the existence has been shown of both a considerable percentage of scientists Who never publish either nationally or internationally, as well as the presence of a small number of highly prolific authors (16). This suggests that it is unreasonable to think in terms of average numbers of papers per scientist in developing countries based on group totals, but rather we should examine the distribution of output within groups of researchers.

Again taking into account the distribution of the Mexican 1981- 1985 SC1 production of papers (14), the UNAM participation was found to be considerably lower in the life sciences than the country as a whole (40% UNAM in medicine and biology compared to 58% Mexican production in the life sciences). Little difference was seen with regard to chemistry (10% UNAM and 8.7% Mexico, respectively) and the percentage figures for physics were identical (24.1 % in both cases). When we compare the latest SC1 1981-1985 figures for Mexico as a whole (14) with those presented for 1973-1975 (17), we notice a twofold

198 Hector DELGADO and Jane M. RUSSEL

increase in the percentage production of papers in physics (24.1% against 12.3%) with a corresponding drop in the medical-biological field (58% in life sciences as spposed to 77% in medicine m d biology).

The small percentage of W M studies in social sciences and humanities the international literature is probably a reflection of the publication

strategy employed by Mexim researchers in this area. $;%rhile researchers in the exact and empirical sciences consider publication in the international literature desirable, their esunterparts in the social sciences m d humanities concentrate the results of their resar tionally published book and journals. The small contribution made by researchers to the agrosciences is explained by the absence of faculties stitution for resarch in this important a r a for developrnent. However, this void is filled by government research facilities and a separate University ddicated to eduation and research in the agricultuml field.

It is interesting to note the di s of scientists working in different fields. While the U published articles wide range ofjourmals with 58 ng only one article, UN .physicists showed a preference for publishing in a selected core of journals. UNAM engineers also seem to publish over a wide range of journal titles. However, much of the research carried out in the engineering fields is financed by industry which often implies confidentially of the re obtained. Even when not confidential, research output is channeled thr documents, such as technical reports and patents, which are not indexed i . Without studyin detail the publication strategies of individual scientists and groups of scientists, it is impossible to h o w the reasons behind these elifferences, although we could speculate that the physicists possibly represent a more homogenous, established resarch community efined preferences for publishing in certain journals. The majorit scientists, on the other hand, apppears to lack unified criteka for the oumal titles in which to publish. This situation is reflected in the wide vamety of titles empfoyed for publication, the majority of which have low impact factom and total citation counts.

Amnachalam and Singh (18) in a study on Israeli scientists mention a large aumber ofpapers published in journals whose impact factom were greater than 2. They quote examples from two areas (Physics, Chemistry and Exact Sciences, and Life Sciences) where the percentage of these pape= is j u t over 50%. In our

re was around 37% taking into consideration al1 areas of UNAM resarch which basically correspond to the above areas mentioned by the Indian authors.

It is important, however, to b a r in mind that bibliornetric indicators, such as used in this study, vary with time and with the discipline under consideration (19). According to Garfield @O), smaller fields like botany and mathematics do not generate as many articles or citations as say, biotechnology or genetics. Also, he points out, in certain fields it may take ten years or more for an article to attmct a meaningful number of citations. The highest 1987 impact factors for al1 joumals indexed in SC1 corresponde$ to the biomedical area and especially to review

Impact of International Publications by Mexican Scientists 199

publications. The non-review journal with the highest impact factor for 1987 was the biological title Ce11 Biolonv with a factor of 22.79, followed by Journal of Neuroscience Research with 22.27. Review journals in the biomedical field reached impact factors of up to 35 for 1987. Physics journals in contrast showed considerably lower impact factors for 1987 than those in the biomedical field which explains some of the differences we found with respect to the publication patterns of UNAM scientists in these two areas.

Further studies on UNAM research output need to be orientated towards defining with precision the quantitative variation in bibliometric indicators which can be attributed to the differences in publication and citation patterns between disciplines. The results could then be applied to improve the accuracy of research output analysis and evaluation by expert committees when assessing the research performance of individual scientists or, as in Our case, groups of scientists, to assign research funding or to select candidates for national prizes or incentives. Not only would we expect to find global differences between disciplines with respect to bibliometric criteria but also variations between different fields within the same discipline? although these would possibly be less significant. As Garfield indicates, the number of authors and journals varies greatly between and within disciplines, as do citation levels and rates (20).

Another fact to be taken into consideration is that the evaluation of research output taking into account the ranking of articles based on journal impact factors does not tell us anything about the real citation rates of these articles. In the study carried out by Schubert and his coworkers to develop a comprehensive set of indicators for 96 countries (14), the impact of Mexican studies in certain subdisciplines, such as astronomy and astrophysics, was found to be superior to that expected from estimations based on the average citation rates per paper of the journals concerned. In general, however, the real citation rates (observed citation rate) of Mexican articles in al1 major disciplines was found to be below the expected citation rates. In other words, Mexican articles were cited less times on average than studies from other countries published in the same set of journal titles.

In conclusion, we can say that UNAM scientists in general publish as first authors in international journals which are not frequently cited by other scientists, thus reducing the visibility and possible impact of studies carried out in this institution. However, indications point towards the existence in the UNAM of an elite group of researchers whose publication strategies include the dissemination of articles in journals with high impact factors and citation rates. However, it is necessary to eliminate variations relating to disciplines before arriving at definite, well-fundamented conclusions concerning the publication patterns of UNAM scientists. We also need to look at publication strategies when the UNAM researcher is not the senior author but rather one of the coauthors, which indicates the existence of collaborative studies between the UNAM and other institutions both in a national and international context. It would also be interesting to analyse further the performance of UNAM scientists in different

200 Hector DELGADO and Jane M. RUSSEL

disciplines, partienlarly with respect to possible variations in output between U Iresearchers and their col1 es wsrking in sther national institutions.

The authors wish to tlhmk Dr. Aida Mbndea for her suggestions csncerning the study md Miguel h g e l L6pez, Gustavo Blmcas and hg6 l i ca Ma. Rosas for techical assistance.

(1) Anderson, J.: New approaches to evaluation in UK research funding agencies. SPSG Concept Papa No. 9. The Science Policv Support Group, London, October 1989 (2) E n g , 9.: A review of bibliometric and other science indicators and their role in research evduation. Journal of Information Science, 13 (1987) 261-276 (3) Frame, J.D.: Quantitative indicators for evaluation of basic research programdprojects. IEEE Transactions on Ennineeska Management, EM-30 (1983) 106- 112 (4) Moed, H.F., Burger, W.J.M., Frankfort, J.G. md Van R m , bibliometric data for the measurement of University research performwce. Research Policv, 14 (1985) 131-149 (5) Mullins, N.C.: Evaluating research programs: measurement and data sources. Science and Public Policv, 14 (1987) 91-98 (6) Nain, F.: Bibliometric techniques in the evaluattion of research programs. Science and Public Policvq 14 (1987) 99-106 (7) ABRC: Evaluation of national performance in basic resexch. ABRC Science Policy Studies No. 1. Advisory Board for the Research Councils, London, 1986 (8) Stolte-HeiskanmenI, V,: Evaluation of scientific performance on the periphery. Science w d Public Policv, 13 (1986) 83-88 (9) Russell, J.M., Correa Noyola, M.S., Garcia Aguila, N., Guadarrama HBrnmdez, J.A. m d Priego Orozco, L.: Research and publication trends of a Latin American veterinary faculty. Interciencia. 12 (1987) 243-244 (10) Russell, J.M. and Galina, C S . : Research and publisking trends in cattle reproduction in the tropics: Part 2. A Third World prerogative. Animal Breedinq Abstracts, 55 (1987) 819-828 (11) Arvanitis, R. and Chatelin, Y.: National scientific strategies in tropical soi1 sciences. Social Studies of Science, 18 (1988) 113-146, (12) Davis, Ch.H. and Eisemon, 331.0.: Mdnstream and non mainstream scientific literature in four peripheral Asian scientific communities.Scientometrics, 15 (1989) 215-239 (13) Tomer, C.: A statistical assessment of two measures of citation: the impact factor and the immediacy index. Information Processinn and Management, 22 (1986) 251-258

Impact of International Publications by Mexican Scientists 20 1

(14) Schubert, A., Glhzel, W. and Braun, T.: Scientometric datafiles. A comprehensive set of indicators on 2649 journals and 96 countries in a l 1 major science fields and subfields 1981-1985. Scientometrics. 16 (1989) 3-478 (15) Velho, L.: Publication and citation patterns of Brazilian agricultural scientists. Social Studies of Science, 14 (1984) 45-62 (16) Gonzdez Blasco, P.: Los que publican y los que no publican en la investigacibn cientifica espaiiola. Interciencia, 5 (1980) 223-230 (17) Frame, J.D.: Mainstream research in Latin America and the Caribbean. Interciencia,

(18) Arunachalam, S. and Singh, U.N.: Where do Israeli scientists publish ? Journal of Information Science, 14 (1988) 221-231 (19) Vinkler, P.: An attempt of surveying and classifying bibiometric indicators for scientometric purposes. Scientometrics, 13 (1988) 239-259. (20) Garfield, E.:How ISI selects journals for coverage: quantitative and qualitative considerations. Current Contents, no. 22, May 28, 1990, pp 5-13

- 2 (1977) 143-147

Hector DELGADO and Jane M. RUSSEL

FIGURE 1

Annual distribution of UNAM research scientists

Nurnber of researchere n 1

FIGURE 2 Annual production of articles

1978 1979 1980 1981 1982 1983 1984 1985

Year of publication

UNAM MEXICO

Impact of International Publications by Mexican Scientists 203

25

20

15

10

5

O

FIGURE 3 Distribution of journal titles and articles

according to the major disciplines

percentage 1

PHYS MED BI0 CHEM ENG EXAC GE0 SOC MUL PSY AGR

disciplines

Journals Articles Total number of Articles= 2192

Total number of journal Utles= 692

PHYS = PHYSICS G E 0 = GEOSCIENCES MED = MEDICINE SOC = SOCIAL SCIENCES AND BI0 = BIOLOGY HUMANITIES CHEM = CHEMISTRY MUL = MULTIDISCIPLINARY ENG = ENGINEERING PSY = PSYCHOLOGY EXAC = EXACT SCIENCES AGR = AGROSCIENCES

204 Hector DELGADO and Jan@ M. RUSSEL

30

25

20

15

10

5

O

FIGURE 4

Distribution of articles In physics fournals

number of journal titles

‘126

1 2 3 rE 5 0 7 8 9 10 11 12 15 10 18 21 23 . . . . . . . . . . . . . . . . .

Number of articles

FIGURE 5 Dlstrlbutlon of artlcles in englneerlng Journals

number 0f journal titles A 1

O b 1

number of artlcles Tolal number of Journal !luos- 06

Total nurnber 01 ortlolwo- 100

Impact of International P

ublications by Mexican S

cientists 205

206 Hector DELGADO and Jane M. RUSSEL

FIGURE 7' Distribution of Journais and artlcles

accordlng to impact factors percentage

Article5 dournale

FIGURE 8 Distrihution of journais and articios

with impact factors <= 1 totals ~ - -

A 131

<=0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1987 imgact factor

1

Journais Articles

Impact of International Publications by Mexican Scientists FIGURE 9

80

BO

40

20

O

accordmg to total number of ~ournal citations Dtstribution of journals and articles

207

percentage -

<=5 10 20 30 40 50 O0 70 O0 100 >100

total number of citations in 1987 (thousands)

Articles Journals

FIGURE 10 Distribution of journals and articles

with total nurnber of journal citations <= 5000

percentage 33.1

--

35

30 25

20

15

10

5

o v , / l

<=500 1000 2000 3000 4000 5000

total number of cita.tions in 1987 - Articles Journals