ENTRAR            

REVIEW

The Hirsch's h-index. An update on the author's evaluation methods and their contributions in scientific publications

Alberto Gálvez Toro,¹ Manuel Amezcua^2
1Coordinador de Investigación. Fundación Index, Granada, España. ²Jefe de B. de Docencia e Investigación. Hospital Universitario San Cecilio, Granada, España. Presidente de la Fundación Index, Granada, España

Mail delivery: Alberto Gálvez Toro. Fundación Index, Apdo. correos 734, 18080 Granada, España

Index de Enfermería [Index Enferm] 2006; 55: 38-43

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Abstract

The evaluation of a scientific-professional author makes sense in a competitive system in which the highest rating has effects in the author's career or in his access to funding. In the evaluation process it is necessary to consider two differentiated dimensions that are usually confused: the measure method and the data source. The first one must be universal and valid for all authors. The second one should be adapted to the author's field of study. J.E. Hirsch has recently developed a new indicator that apparently resolves the validity problems of the calculation systems known to date. This indicator is called h-index and differs from one scientific community to another, that is, from discipline to discipline, so it is necessary to develop a research that defines its values in all branches of learning (h-index per discipline).
The value of number h represents the author's scientific biography. It doesn't measure immediacy, but long periods of time. The scientific-professional activity is biographic, therefore it was necessary to create an indicator that measured the career and the route. An acknowledged author in his field will have a higher h-index at the end of his professional life than at his beginnings independently of the journal in which he publishes or in the means used to disseminate his contributions.

Introduction

    Within the framework of scientific publications, the author is the person that publishes in scientific-professional press, no matter whether he practices or not the profession of scientist. The author evaluation is the process that allows the assignation of a certain mark which helps locating him compared to other authors. The evaluation intends to discern if a certain author A is better than other author B and therefore should receive a higher mark than the second one.^1-3
    The evaluation makes sense within a competitive system in which the highest position in the assessing scale implies a certain benefit. That benefit is translated into a profit for the author, the institution to which he belongs, or his work's research line. In other words, the author evaluation is a value within a market. He is quoted like the enterprises are quoted in the Stock Market. For the author himself, the result of the evaluation may mean the recognition of his contributions and the funding of his research lines, his access to a position within a university or institution, etc. But it can also imply the opposite. It only depends on the criteria adopted for the evaluation. In other words, it only depends on the measurement tools.^4-6
    Within the field of science, it is said that a measuring tool must be valid, that is, it must represent the reality under measurement, or it must give a close approximation of the reality of the issue being measured. If an author's evaluation tool is not valid, it will be stated that the evaluation done does not represent the reality under measurement. This possibility represents a serious problem. The evaluation tool used must be universal and equal for every author. In other case, an outrage against a principle of science would be committed. The evaluation tool must avoid bias or deviations, and must be applicable in different contexts preserving its equal validity on each.
    Which evaluation tools do we have at our disposal at present? There are four fundamental evaluation methods: the measurement of the impact index of the journal publishing the author's work, the quantification of the quality and number of references to the author's work, the number of articles (or other computable scientific contributions) published by the author, and the pair-based evaluation of the author's contributions when he competes for an academic position, for a research position or for funding.^4,5,7
    Recently, Hirsch⁸ has proposed a new index, the so-called factor h, index h, or following Hirsch's own denomination, the h-index. Searching simplicity, science has since long time ago looked for the golden ratio, a quantitative method, a unique and straightforward indicator valid for all the branches of science and disciplines; an index capable of representing the value of an author and its contributions.⁹ We will review all the evaluation indexes related up to now. Hirsch might have defined a tool close to the golden ratio.

Methods to calculate de impact of an author

a) The journal impact index or indirect method. Measuring the importance of an author throughout the impact index of the journals on which he publishes, is a more than uncertain method to evaluate him. The impact index of a journal measures exactly that, the importance of the concrete journal within its group of equals.⁷ The impact of a journal of Physics is only comparable to the impact of another journal of Physics. This can be extrapolated to different disciplines like Mathematics, Medicine of Nursing.
    The journal impact indexes used in the evaluation processes are the result of dividing the number of references made to the journal under analysis, by the number of articles published in that journal. These indicators have many limitations, and well known procedures to alter their value do exist.⁴ They will not be analyzed in this work, but it is interesting to remark that their value depends on the criteria of inclusion of the source articles (those articles accepted to be published in the journal, which are the articles whose references are computed). Impact results are very different if all the articles are computed, if only the reviewed ones and their original are computed, or if only the ones including bibliography are computed. The most well-known impact index is the Impact Factor Thomson Scientific that is calculated using the references received by each journal on the two years previous to the year of evaluation, divided by the number of articles published in the journal during those two years. The Institute of History of Science Lopez Piñeiro¹⁰ follows an analogue procedure.
    The impact indicators of the Fundación Index¹¹ are also remarkable. Four basic indicators are calculated: the immediate impact, the immediacy index, the historical repercussion and the historical repercussion without self-references. Other strategies to calculate repercussion and impact can be found in the studies of the Superior Council of Scientific Researches.^12-14
The impact of a publication is an indirect measure of its author's repercussion.^2-4 It is commonly accepted that journals with a higher impact index have more rigorous control mechanisms on the materials they publish.⁴ Nevertheless this only means that the work published has been evaluated following more strict criteria. Nothing else. Any other conclusion is a speculation.^15,16 It is true that there are studies which have found a relation between the journal impact and the quality of the articles published in it.¹⁷ However, does this represent the author impact or his repercussion? The current answer is no, this is not a valid method to evaluate an author. In any case, it can be useful for the journal publishing house to sell more or less issues or subscriptions.

b) Measurement of the references to the author's work. Evaluating the importance an author based on the existing references to his work is a very interesting method.^1,2,4,7 At first sight it seems the ideal method of evaluation: is there a better way to evaluate an author than computing the times he has been referenced? Nevertheless, this method also has its limitations. We will mention only two of those limitations.
    Let's imagine an author nurse having been referenced 45 times during the last 5 years. It can be concluded that her works have had repercussion within the concerned community. Nevertheless, when the references are analyzed, it is found that all of the references except two of them are references to a nursing guide on diagnosis. In other words, the author has only received references to three of her works. During the last 5 years, she has only published one article and one guide. The second limitation of this reference method is due to the variety of ways to reference her or him. It is common for an author to be referenced in more than one different way. For example, Luis Martínez Andrade could appear referenced as Martínez, LA; Martínez LA; Martínez Andrade, LA; Martínez Andrade L; Martínez Andrade Luís Andrés. The different ways to reference the author make necessary many diverse searches in the correspondent databases. The situation becomes more complicate when the authors have family names as frequent as García or Fernández.
    This problem can be solved by searching the publications of an author. If we are looking for the references to López's works and we know he has produced an article titled "Adjusted bibliographic impact", it is enough to search those terms to know the times this article has been referenced. This procedure should be followed with all of his works to obtain the number of references done to each of his works. The sum of all theses references per work is the global number of references made to the author's work. This procedure is very slow and normally only the author himself can do it, as he is the one who knows his whole scientific production. The advantage of this method is the fact that it permits to know the work's impact: the references that each contribution has independently obtained.

c) The number of works published by an author. The number of articles published by an author does not give too much information either. The case is identical to the case of the bibliographic impact of a journal. If the author has published many articles, the only conclusion that can be extracted is that he has been a very active writer.^1,2,7 Nevertheless this measurement does not give us any information about the effect of his contributions. If we add the fact that his works have been published in impact journals of a certain discipline, the measurement only points out that the author has passed the journal's process of selection and sifting. But we do not know if his contributions have any repercussion.
    It is usual to calculate an indicator including citations and published works to overcome this limitation. The author's impact is then defined by the number of references he has received on the works he has published. This measurement procedure has many limitations. If the author of the guide on diagnosis mentioned above has published along her whole scientific career 14 contributions that have been referenced 45 times, the result of the division would be (45 references/14 contributions) 3,2. This 3,2 would be her author's impact, but the reality is that only 3 of her works have been referenced, and the most of her citations (43 over 45) are accumulated on an unique work (a nursing diagnosis guide). An author having received a total of 300 citation on a total of 95 publications would have a similar impact (300/95=3,1). 95 Publications are many publications and 300 references are many references as well. The author's impact does not provide information on the references' distribution along the works or contributions. It is not the same having 300 references on 10 works than having 300 references on 50 works. The trajectories are different in both cases: at least in theory, more years of work are needed to publish 95 works than to publish 14. In spite of this fact, both authors have similar impact (3,2 and 3,1).

d) The pair's evaluation of the author's contributions or qualitative evaluation. In principle, the candidate's evaluation based on the revision of a commission (pair's revision) seems to be a good evaluation method.^1,2,4,7 It would be nice if this corresponded to reality. When an author submits himself to the judgment of a commission he normally does it under the principles of a competitive concurrence in order to obtain something specific: a research or academic position, or funding for a research project. Due to the fact that it is not possible to create blind conditions for the evaluation of the candidate's professional trajectory, without the precise tools, there are no warranties of an objective evaluation made by competent evaluators free of conflicts of interests and power.
    An example of the perverse effects that can be produced by a deficient evaluation is the separation from the financing circuits of certain groups and areas of knowledge. This example represents at least what has been happening in the area of nursing with regard to the main governmental agency of health research in Spain: the Health Institute Carlos III (ISCIII), whose evaluation criteria are highly medicalized. As a consequence of this medicalization, the contributions of nurses have been invisible to a great extent. The memory reports of the Institution and the publications of the Boletín Oficial del Estado^18,19 show that nurses receive a very poor financing even though they represent the second most numerous professional group, following doctors, within the National Health System. We must remark that nurses demand few research projects to the FIS, but this fact should represent itself something to analyze and worry about, especially because nurses are a professional group with a scientific production objectively important as the bibliographic databases linked to their geographic and disciplinal space show. In terms of results, the ISCII funding for nurses represents, at the most and having a favouring deviation, the 0,15% of the contributions in scientific productions collected in CUIDEN.²⁰ Nowadays the fact that the ISCIII funds or not nurses does not alter substantially the disciplinal knowledge produced in the country, being this lack of effect of the potential funding an even more worrying factor. Therefore, the qualitative evaluation is not exempt of problems.

e) The h-factor, The Hirsch's h-index. The h factor or h-index is a number representing the author's weight in the scientific community. It measures the author's repercussion in a very peculiar way, not related to the indicators described above. J.E. Hirsch, creator of the h factor, expresses clear and concisely what he has found: a simple and valid method to know the author's repercussion: "I would like to propose a single number, the 'h-index', as a particularly simple and useful way to characterize the scientific output of a researcher".⁸
    The h index is the number applied to a researcher who has h works which have been referenced at least h times. For example, h=5 implies that the author has 5 articles which accumulate 5 or more references to them, but this author does not have 6 publications having been referenced 6 or more times.^8,9 This measurement index has some limitations. Among others, it depends on the size of the reference scientific community and it varies from one discipline to other. Its great advantage is the fact that it is a number which takes into account the magnitude of time and can evaluate big periods of time considering the whole scientific career of any author. It is recommended to use it to analyze periods of time bigger than ten years. The bigger the evaluated period is, the more exact and fair the evaluation made of the author's trajectory will be.
    In order to make a theoretic demonstration of utility of the h index, Hirsch focused on the Physics. The most prestigious physicists of the world have an h index higher than 60 (they have 60 articles which accumulate at least 60 references each). A successful physicist will have at least an h index of 20 during the last twenty years. An h value of forty can only be found for researchers belonging to big and elitist research centres. An h value of sixty in twenty years or an h value of ninety during thirty years characterizes a unique and exceptional researcher. The most of the Nobel prizes during the last 20 years accumulate an h value between 35 and 39. The concession of this award is no fluke.^8,9 Hirsch always used the Thomson Scientific Database to calculate the h index.
    The h index is the golden ratio for an author evaluation. Nevertheless some questions should be pointed out although they could be redundant:
    1. The h index measures particular works belonging to particular authors. It does not take into account languages or specific journals. Any reference has the same value for an author.
    2. Its standard value for each discipline is not known. Researches to calculate its adapted value for each discipline should be done. The h value for nursing cannot be comparable to the h value for medicine, nor the h value for the different scientific and linguistic communities. For small communities, the h index value will always be smaller.
    3. The h factor measures the author's trajectory, his biography as scientist. The h values for the physicists and their Hirsch classification are based on periods of time between 10 and 20 years.

Data sources

    So far, the measurement methods and the different indicators used to calculate the author's impact have been discussed. The measurement methods have nothing to do with the data sources. The first ones, except in the case of a qualitative evaluation, can be defined as more or less complex mathematic processes consisting on the addition or division of numbers. The data sources are the documentary resources permitting to obtain the numbers (data) to make the calculations.
    The documentary resources are databases, questioning systems to which the question of how many times an author has been referenced can be posed. The author's citations number is returned by the documentary resource in a more or less incomplete way. There is no universal database in the world able to answer with validity all the questions a scientific author wants to pose: How many times have I been referenced? Who has referenced my work? There are, nevertheless, different data sources that give partial answers to the author who should become the researcher of his own work to know its impact and demonstrate it. Each database has some selection and inclusion criteria which make it incomplete and introduce bigger or smaller slants and deviations when answering queries depending on the considered discipline (or even the on the language used to make the query).
    Bellow we will refer the main data sources to answer the two basic questions each author poses himself, focusing the interest on Health Sciences (which are not biomedical sciences).

a) Thomson Scientific Products. Thomson Scientific is a North American knowledge management enterprise. Its products are the most known across the world. It is responsible of creating the Impact-Factor, and the set of products surrounding the environment of Thomson INSI. When counting on a big database it is very simple to use it to do research on knowledge production and management. It has a serious idiomatic deviation. The most of the publications it takes in are created in English.^1-4,7 Since year 2005 its policy of inclusion of scientific journals has started to change, due to strategic and economic reasons. Thomson Scientific is an enterprise looking for new markets and benefits increase. Spanish medical journals are been included in the JCR as part of this expansion strategy. It is not appropriate to get into the small print of the benefits Thomson obtains with this inclusion of Spanish medical journals, but it is clear that this inclusion corrects the language deviation, and has an even more important effect: the Spanish authors and journals increase their impact. With this inclusion, the national research statistics based on the analysis of the Thomson ISI products will get their impact values improved. I will then be stated that there is more and higher quality research in Spain, with a higher international impact. The fact is that having an adequate number of national journals placed there "within the internationality", the Spanish research will have done an important virtual turnabout. The research policies in Spain are following that direction, with the subsequent ethic and legal important dilemmas associated to the strategy. The national organisms with competences to evaluate knowledge, force authors to publish their works in journals related to this North American enterprise under the threat of "not being positively evaluated".
    In any case, the products of Thomson Scientific are an excellent data source to know if an author having published in English-speaking journals has been referenced, and by whom.

b) Scielo. Scientific Electronic Library Online. Scielo is the biggest platform for whole-text scientific journals within the Latin American research space.^21-29 It is a whole-text library containing tens of journals. Each participant country has developed an independent platform, integrating all of them afterwards as a whole. With no doubt, the most complete platform is Scielo Brazil²³ which is the only one offering the possibility to make queries on references. Its user interface needs still some refinements to be done, but it constitutes an interesting project as it permits to measure the impact of the authors' contributions within the Brazilian environment.

c) Academic Google.  Academic Google30 is a product created by Google^®, usable for free. Although it needs a more precise user interface it provides valid information about the references received by an author or an author's work. Since the beginning of 2006 until now its results have been substantially improved. We can consider that Academic Google has universal coverage, with two qualifications: the author under query must exist within the Internet, and the query's coverage will be as extensive in time as the history of the Internet itself, that had its outburst of development during the 90's, permits.
    Academic Google is an interesting data source for an author to calculate the number of references he has received. If the given instructions³⁰ are correctly followed, the results can be very impressive. Its main advantage is the fact that it has no restrictive criteria of inclusion or exclusion. If you exist in the Internet and have been referenced by someone, Academic Google will find it. Naturally, this fact ends with the hegemony of Thompson Scientific.

d) CUIDEN citation. The CUIDEN citation index¹¹ permits to know the production, consumption and repercussion indexes of the Nursing Journals within the Latin American scientific research space. The database CUIDEN citation,¹ a much more advanced whole-text version, can calculate the times an author (a publication or a journal) has been referenced within the Latin American scientific research space. In addition, it offers information about the source publication and the referenced publication under analysis. It answers the main two questions that every author poses: which work of mine has been referenced and who has referenced it. It collects information previous to 1996 although the biggest part of the source publications has been compiled since 2000 up to now.
    Let's analyze the CUIDEN citation index utility calculating the number of references of an author and his h-index. The CUIDEN citation database permits to know that until the 27^th of December of 2006, Dr. Siles González (from the University of Alicante) has been referenced 175 times in 96 source publications within the Latin American scientific research space.His Hirsch's h-index has a value of 6 (he has 6 publications that have been referenced at least 6 times). Eva Mª Gabaldón Bravo, a member of his team with a shorter research trajectory, has been referenced 19 times in 17 source publications. Her Hirsch's h-index has a value of 2 (she has at least two publications which have been referenced twice at least). The detailed analysis of her case shows that one of her works has been referenced six times, five other works have been referenced twice and three of her works have been referenced once.
    What do these values show within the context of the data source used? First, they point out that the h-index has lower values within the Nursing Latin American scientific research space, than it had in the field of Physics used by Hirsh to define the measurement index. This difference is due to two facts: the data source evaluates periods shorter than 10 years, and the reference scientific community is small in terms of production and number of journals. It is probably difficult to find authors with an h-index higher than 10 in the actual conditions within this area of work. There is still some analysis to be done about this question. The second conclusion to obtain when analyzing the h-index values of the example is that Dr. Siles has a research trajectory different to E.M. Gabaldón's.  He has an h-index of 6 (6 references to 6 works = 36); E. M. Gabaldón has an h index of 2 (2 works referenced twice = 4). The ration between both indexes is geometric, not lineal, having Siles a bigger and superior scientific biography compared to Gabaldón. This comparative result is corroborated by another fact: Siles has got 79 publications indexed in CUIDEN database since the beginning of the 90's. Gabaldón has got 16 publications, the first of which was published in 1998. She is a young author while and he is a mature one.  With the years, it is expected for both authors to increase their h-index value, the number of references received and the number of publications.
    The calculation the h index has many other applications within the CUIDEN database. One of them is for example to identify the most influent international research lines or the most influential authors for the Nursing development within the Latin American scientific research space. For example, L. Aiken has got 48 references in CUIDEN citation and an h index of 5. With this information we can conclude that his works about Nursing teams, satisfaction, morbidity-mortality and magnetic hospitals have great importance. Another example: the numerous references to authors such as J. Habermas, M. Foucault or T. Kuhn show a scientific interest on the Critical Theory, the Structuralism or the Epistemology respectively.

e) Manual Search: the original document as direct data source. It might seem strange to reference the manual search in this work. It is a valid method, especially in the case if of those disciplines, geographic, linguistic or knowledge areas which do not have any other data sources (although Google Academic will always give some results, as well as CUIDEN citation, as they do not exclude any kind of reference).
    The manual search is the search based on the systematic revision of the bibliography of one or several periodic or nor periodic publications, to find references to an author. This work is usually done by the author himself, and it is done easily when the reference scientific community is small and the author is very much specialized on a certain topic. It will be enough to review the articles which are yearly published on that certain topic, to know if the author has been referenced. M. Richart, for example, has got a web page where he gathers 101 source publications with references to 21 of his publications.³⁵

Recomendations

    The references to an author constitute a very important indicator to evaluate him. The value of the Hirsch h-index includes supplementary information: the author's trajectory. Every reference should be equally evaluated. In the end, it has been demonstrated that neither the journal impact, nor the number of references/publications, nor any other measurement tool described, are determinant to evaluate an author. We must defend the usage of the h index as measurement tool. Concerning the database for the queries, there is no particular one better than the rest. Any database helping to calculate the h index is useful, even the manual search.  Each author should exploit the different sources to show that his contributions have had an impact and that he is competitive when compared to others.
    Forcing to publish in Thomson Scientific's journals constitutes an excess from all points of view (ethic, strategic and legal). It means to confuse calculation methods and data sources. Denying the validity of reference because it is published in Spanish constitutes a negligence which increases the dependency on external products and attempts against the innovative technologies and applications.  The researches on Bibliometry within the Latin American scientific area are mature enough not to go backwards.^31-35 Societies make progresses and disciplines are developed due to the creation, application and implementation of self-evaluation and managing mechanisms that ease the planning towards future.³ Spanish is the third most spoken language in the world, after Mandarin Chinese and English. There are 380 millions of Spanish speakers, 45 millions of people who chose Spanish as first foreign language. Brazil and the USA increase exponentially their Spanish speakers'. And, what becomes more important, the 15% of the world's PIB speaks Spanish. The Science is also communicating in Spanish. Diversification is the horizon of the knowledge management policy. It is a shame that some governmental agencies move against the global trends. Nurses will continue using sources of their own scientific space and other areas (Spanish, English, Portuguese, etc.; nursery, physiotherapy, occupational therapy, medicine, etc.) to complete the cycle of knowledge.^36-38 Future research must be oriented towards the elaboration of an h-index for the Latin American scientific research space (in the same way we already have indexes measuring the most productive and referenced authors)³⁹ in order to achieve results similar to those obtained by Hirsch when analyzing the Physics. The next step, already posed, is to determine the social impact of the research done by nurses. This will be the definitive jump from considering the bibliometry as scienciometry, to analyzing it as part of the sociology of knowledge and its social implications.

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