 Research
 Open Access
Ranking MEDLINE documents
 Célia Talma Gonçalves^{1}Email author,
 Rui Camacho^{2} and
 Eugénio Oliveira^{3}
https://doi.org/10.1186/167848042013
© Gonçalves et al.; licensee Springer. 2014
 Received: 17 May 2013
 Accepted: 9 March 2014
 Published: 19 June 2014
Abstract
Background
BioTextRetriever is a Webbased search tool for retrieving relevant literature in Molecular Biology and related domains from MEDLINE. The core of BioTextRetriever is the dynamic construction of a classifier capable of selecting relevant papers among the whole MEDLINE bibliographic database. “Relevant” papers, in this context, means papers related to a set of DNA or protein sequences provided as input to the tool by the user.
Methods
Since the number of retrieved papers may be very large, BioTextRetriever uses a novel ranking algorithm to retrieve the most relevant papers first. We have developed a new methodology that enables the automation of the assessment process based on a multicriteria ranking function. This function combines six factors: MeSH terms, paper’s number of citations, author’s hindex, journals impact factor, author number of publications and journal similarity function.
Results
The best results highlight the number of citations and the hindex factors.
Conclusions
We have developed and a multicriteria ranking function, that contemplates six factors, and that seems appropriate to retrieve relevant papers out of a huge repository such as MEDLINE.
Keywords
 Ranking
 Text mining
 Machine learning
Background
It is very important for researchers to be aware of the relevant scientific research in their scientific area of knowledge. However the volume of scientic and technical publications in almost all areas of knowledge is growing at a phenomenal rate. Most of these publications are available on the Web. Thus, accessing the right and relevant information amidst this overwhelming amount of information available in the Web is indeed of great importance, albeit difficult in most cases [1].
When trying to find relevant publications, researchers turn to the well known traditional keywordbased search engines, which returns, as a result, a huge list of publications, that usually include a large number of irrelevant ones [2].
To tackle this problem, research in Text Mining and Information Retrieval has been applied to literature mining in order to help researchers to identify the most relevant publications [1, 3].
We have developed a Webbased search tool, BioTextRetriever, to find relevant literature associated with a set of genomic or proteomic sequences. BioTextRetriever uses Text Mining and Machine learning techniques. Machine Learning techniques are used to automatically train a classier that learns with the papers associated with each set of input sequences. The learned classifier is then used in the process of retrieving relevant papers from a larger repository such as MEDLINE^{a}.
BioTextRetriever also organizes the papers selected as relevant by the classifier using a ranking function. We have proposed and evaluated a ranking function that combines MeSH terms^{b}, paper’s number of citations, author’s hindex, journals impact factor, authors number of publications and journal similarity factor^{c}.
In the rest of this article we first present the related work (Section “Stateoftheart”). In Section “BioTextRetriever architecture” we describe the system’s architecture. The methodology used for the automatic classifier construction process is described in the Section “Methods”. The ranking process is presented in Section “The ranking process”. The proposed ranking function is explained in detail in Section “The ranking function”. The experimental evaluation of the ranking function is described in Section “Choosing the ranking function coefficients”. We discuss the results of such experiments in Section “Results and discussion”. Finally we draw the conclusions in Section “Conclusions”.
Methods
In this section we describe the methodology we used for designing BioTextRetriever.
Stateoftheart
The areas of Information Retrieval, Text Mining and Document ranking are in fact very active research areas. We now reference and comment on work done in those areas that is related to ours.
As text classification is concerned there as a lot of different approaches, including approaches that use Machine Learning. As far as we know there is no previous work that dynamically constructed a text. The main reason is that existing approaches (as is usual in classification problems) require the instances to be preclassified by an oracle. In our application, when the system runs, we have no access to an oracle to preclassify the instances. We have taken advantage of fact that there are a few papers associated to the biological sequences stored in NCBI. We take those papers to be the relevant papers and automatically collect the instances for the alternative class (irrelevant papers) and, in this way assemble automatically a data set.
In [4] the authors use machine learning to order documents by popularity, or the predicted frequency that an article is viewed by the average PubMed user. The authors claim that the identified method for learning popularity from clicking through data shows that the topic of an article influences it’s popularity more than it’s publication date. Opposite to our approach [4] method relies on available measures of popularity collected during the use of the system. As seen below our approach relies only on information that is naturally part of the NCBI data bases and not the result of user interactions.
Deng et al. 2012 [5] proposes a unified model, PAV, for ranking heterogeneous objects, such as papers, author, and venues. PAV explores object ranking in bibliographic information where objects are papers, authors and venues. In PAV the bibliographic information network is represented by a weighted directed graph, where a vertex stands for an object, an edge stands for the link between objects, and a weight over an edge stands for the degree of contribution that one object devotes to the importance or reputation of the corresponding object sharing the same edge with the object. The rank (importance or reputation) of an object is the probability that the corresponding vertex is accessed by random walk in the PAV graph. The authors claim PAV is an efficient solution for ranking author, paper, and venues simultaneously. According to their method, the importance or reputation of an author is influenced by his coauthors, his papers, and the venues that published his papers. The importance or reputation of a paper is influenced by its authors, its venue, and the papers that cited it. The importance or reputation of a venue is influenced by the papers that it published and the authors who had papers published by the venue. PAV model transforms the problem of ranking objects into the problem of estimating probability parameters. For estimating probabilities the authors developed an algorithm based on matrix computing. The authors claim their algorithm could be ran efficiently by proving that the underlying computing method is convergent.
The authors in [6] present an approach that jointly ranks publications, authors and venues. They first constructed a heterogeneous academic network which is composed of publications, authors and venues. A random walk over the network was performed hence yielding a global ranking result of the objects on the network. The mutual reinforcing relationship between user expertise and publication quality was based on users bookmarks. The authors claim that their experimental results with ACM data set show that their work outperforms all other baseline algorithms, such as Citation Count, PageRank, and PopRank.
In this paper [7], the authors present three different prestige score (ranking) functions for the contextbased environment, namely, citationbased, textbased, and patternbased score functions. Using biomedical publications as the test case and Gene Ontology as the context hierarchy, the authors have evaluated the proposed ranking functions in terms of their accuracy and separability. They concluded that textbased and patternbased score functions yield better accuracy and separability than citationbased score functions.
The paper [8] proposes an iterative algorithm named AP Rank to quantify the scientists’ prestige and the quality of their publications via their interrelationship on an author paper bipartite network. In this method a paper is expected to be of high quality if it was cited by prestigious scientists, while highquality papers will, in turn, raise their authors’ prestige. AP rank weighs the prestige of quoters more than the number of citations. Given that old papers will have more chances to accumulate more citations than recent works the authors proposed a timedependent AP rank (TAP rank). According to the authors the main advantages of AP rank are that it is parameterfree; it considers the interaction between the prestige of scientists and the quality of their publications and it is effective in distinguishing between prestige and popularity.
The authors in [9] determine whether algorithms developed for the World Wide Web can be applied to the biomedical literature in order to identify articles that are relevant for surgical oncology literature. For this study the authors have made a direct comparison of eight algorithms: simple PubMed queries, clinical queries (sensitive and specific versions), vector cosine comparison, citation count, journal impact factor, PageRank, and machine learning based on polynomial support vector machines. As a result of this study they concluded that the mentioned algorithms can be applied to biomedical information retrieval and that citationbased algorithms were more effective than non citationbased algorithms at identifying important articles. The most effective strategies were simple citation count and PageRank and citationbased algorithms can help identify important articles within large sets of relevant results.
In [10] the authors propose a ranking function for the MEDLINE citations. This function integrates the Citation Count Per Year and the Journal Impact Factor which are two of the factors that integrate the ranking function we have developed. The goal of this work is to present to the users a reduced set of relevant citations, retrieved and organized from the MEDLINE citations into different topical groups and prioritized important citations in each group.
The referred work uses graphs, existing webbased algorithms, and some propose a more specific ranking function. We may conclude that to choose an existing ranking algorithm or to develop a new ranking function depends on the work to be applied and on what the researchers want to achieve. In our case we decided to develop a multicriteria ranking function in order to satisfy all the issues we believe to perform better for ranking the MEDLINE papers.
OKAPI BM25 [11] uses a bagofwords retrieval function that ranks a set of documents based on the query terms appearing in each document, regardless of the interrelationship between the query terms within a document.
Cosine Ranking [12] finds the K docs in the collection “nearest” to the query ≥ K largest querydoc cosines. It allows the documents to be arranged according to their relation with the query.
In both [11, 12] a direct comparison is made with the query made by the user. In our case there is no query to compare with, we have a set of relevant documents (associated with the similar biological sequences). With the approach we have taken we can generalize the common attributes of those relevant papers and use them (in the form of a classifier) to search for other relevant papers.
BioTextRetriever architecture
The sequences, provided by the user, are used as seeds to fetch similar sequences in the NCBI web site. This is the first task (step 1) performed by the tool. Along with the similar sequences, the NCBI web site stores a set of paper references associated with the sequences. In step 2, the references of the papers associated with the similar sequences are retrieved. For each of those papers the following information is retrieved from MEDLINE: PubMed unique identifier (pmid), journal title, journal ISSN, article title, abstract, list of authors, list of keywords, list of MeSH terms and publication date. Considering the scope of this research work we take into account paper references that have an abstract available in MEDLINE.
We take this initial set of relevant papers as the “positive examples” and then we add an equal number of “negative examples”. We have done a set of experiments to determine a proper way of collecting the negative examples (details in [13]). The negative examples are randomly collected among the MEDLINE papers that have MeSH terms in common with the positive examples. After step 2 we have a “proto data set”. The proto data set is subject to Text Mining preprocessing techniques and converted into a data set in step 3. The preprocessing techniques applied are: Handle Synonyms; Stopwords removal; Word validation using a dictionary and Stemming.
Step 4 is one of the most important stages of our work and consists in the dynamic construction of a classier using Machine Learning techniques.
The resulting classier is used as a filter to collect an extensive list of relevant articles from the whole MEDLINE (done in step 5). The final list of relevant papers is usually very large. We, therefore, need to order the by relevance. That is the goal of the last task carried out in step 6. For that last task we have developed a ranking function that will be described in detail below.
From sequences to papers
The main part (step 4) of BioTextRetriever’s architecture (see Figure 1) is the construction of a classifier which output will be ordered by a ranking procedure, which is the aim of this paper.
The core of BioTextRetriever is the automatic construction of a classifier, acting as a filter, capable of selecting the relevant papers among the whole MEDLINE bibliographic database. “Relevant” papers, in this context, means papers related to the set of sequences that were provided as input to the tool. We now describe the methodology to construct such a classifier as well as a set of experiments that support the choices we have made.
Before we address the classifier construction issue, we must address the construction of the data set (step 3). For most of the learning algorithms we must provide positive and negative examples, in our case both relevant and irrelevant papers.
In [13] we have empirically evaluated three different ways of obtaining the irrelevant papers, which we named as: NearMiss Values (NMV), MeSH Random Values (MRV), and Random Values (RV).
To make the distinction between relevant and irrelevant papers more clear we have established a “no man’s land” zone^{e}. This “no man’s land” zone is represented in Figure 2 by the gray region. The papers associated with the sequences in this gray region are discarded. In Figure 2 the box on the right represents the “not so near” sequences that provide the “nearmisses” papers.
The [13] study suggested that generating randomly the negative examples produces better results.
Classifier construction process
In the alternative adopted we have used the whole data and the use of an ensemble of T basic classifiers (C_{ i }) when each basic classifier uses the whole data. For the Ensemble we have evaluated three well known algorithms: AdaBoost, Bagging and Ensemble Selection. Different “ensemble parameters” were tested in the experimental evaluation of this alternative is described in detail^{f}.
Data characterization
Characterization of data sets used to assess the Ensemble algorithms (AdaBoost, Bagging and Ensemble Selection
Data Sets  Number of  Positive  Negative  Total 

Attributes  Examples  Examples  Examples  
S12  1602  128  128  256 
H11  1461  120  120  240 
ERYT21  1592  118  118  236 
HYP11  1706  130  130  260 
HYP21  1944  194  194  388 
BG11  1546  97  97  194 
BG21  1631  115  115  230 
BG31  1859  149  149  298 
LUNG21  1535  120  120  240 
Evaluating the results
As expected through the literature [15] ensemble learners have a higher and uniform performance than base learners.
The ensemble is made using the WEKA’s ensemble classifiers Bagging, AdaBoost and Ensemble Selection.
Ensemble’s Accuracy results
Data Sets  AdaBoost  Bagging  Ensemble Selection 

S12  99.2 (1.7)  98.1 (3.3)  97.3 (3.7) 
H11  99.2 (1.8)  99.2 (1.8)  96.3 (5.0) 
ERYT21RV  95.4 (4.2)  95.8 (3.9)  93.3 (4.5) 
HYP11  91.2 (4.1)  90.0 (6.3)  91.2 (4.1) 
HYP21  95.1 (1.9)  95.1 (3.3)  89.7 (6.5) 
BG11  93.8 (3.4)  94.3 (4.6)  94.3 (5.1) 
BG21  95.7 (3.6)  94.4 (4.6)  93.9 (5.1) 
BG31  93.9 (3.2)  91.6 (4.8)  92.3 (5.0) 
LUNG21  93.8 (4.1)  92.5 (4.7)  92.5 (4.3) 
Overall Average  95.3 (3.1)  94.6 (4.1)  93.4 (4.8) 
As a global result we can see that all algorithms have in general very good performance, well above the majority class predictor (the ZeroR result are around 50%). The three ensemble learners used (Bagging, AdaBoost and Ensemble Selection) according to the tstudent test (α=0.05) have no statistically significant difference. Thus we conclude that either one can be chosen.
The ranking process
We will first present a summary of the tool’s steps up to this point. The user provides a set of sequences, to which the NCBI BLAST tool [16] associates a set of similar sequences. Each of these similar sequences has in turn a set of papers associated. We collect all the papers associated to these similar sequences to build the “relevant papers” part of a data set. The data set is completed by adding a set of “irrelevant papers”, through the Random Value method described in Section ‘BioTextRetriever architecture’. With this data set as input to a machine learning algorithm a classifier is constructed. This classifier will be used, later in the process, to filter the relevant/irrelevant papers from MEDLINE.
From the set of relevant papers associated to the input sequences we extract a set of MeSH terms appearing in these papers. We then search the MEDLINE database for the papers that have the MeSH terms in common with the set’s. Since the whole MEDLINE has a huge number of papers, we use the MeSH terms as a filter to reduce the initial volume of data to a reduced potentially relevant number of papers. Thus we create a MEDLINE sample with papers for classification, that are, somehow related to the introduced sequences because they possess the highest number of MeSH terms in common with the relevant papers we have so far. With this we construct a data set of papers to be given to the classifier for classification. After classification we get a set of papers classified as relevant. To this data set of relevant papers we add a random set of 50 papers, extracted from the set of papers associated to the similar sequences, associated themselves in the first step with the initial sequences entered by the biologist. This procedure was only performed in the experiments intending to decide on the components of the ranking function. The procedure is not performed in the current use of the tool.
The ranking function
Despite the potential relevance of the papers returned by the BioTextRetriever, we need to point out which are the most important papers to present to the user first. A ranking is an ordering of the documents that should reflect their relevance to a user query [12].
The traditional methods for ranking web pages are not suitable to rank scientific articles, since the traditional ranking algorithms (PageRank, Hits, Salsa and Ranknet to name a few) are based on the number of links to a web page. Besides this reason, the existent algorithms do not take into account the items we believe are the most important to consider in the ranking such as the MeSH terms, the number of publications of the authors, the number of citations of a paper, the hindex of the author of a paper, the journal impact factor and the Journal Similarity Factor. None of the mentioned algorithms involves a function that contemplates the mentioned items. Thus, we have proposed a function that reflects the specific criteria we believe to be the best to use in this case. We propose an integrated ranking of MeSH terms, PubMed number of citations, author PubMed hindex, journals impact factor, authors’ number of PubMed publications and the journal similarity factor where the relevant papers were published. The combined use of several indicators that give information on different aspects of scientific output is generally recommended [17].
As explained in the previous section we have used a sample of MEDLINE with papers that have the highest number of common MeSH terms, that we believe to be the best one to use with the relevant papers associated with the introduced sequence(s).
After classification the resulting^{g} paper references are ordered by the following ranking function:
C 1∗M e S H + C 2∗C i t a t i o n s + C 3∗h i n d e x + C 4∗I F a c t o r + C 5∗P u b + C 6∗J S F a c t o r where:

MeSH is a weighted sum of MeSH terms in common with the papers associated with the introduced sequence(s);

Citations is the paper’s number of citations in MEDLINE;

hindex is the highest hindex among the authors’ hindex;

IFactor is the Journal Impact Factor;

Pub is the number of publications of the author with the highest number among them all;

JSFactor is a weighted sum of the number of papers published in a journal that has papers associated to the introduced sequence(s).
Coefficients C1, C2, C3, C4, C5 and C6 may vary between 0 and 100 and their sum must be 100. The set of experiments to determine the values of these coefficients is described in detail in Section ‘Experimental settings’.
Besides the information contained in MEDLINE, we have added some extra information to the Local Data Base (LDB). Besides the Journal Impact Factor all the terms in the ranking function (number of MeSH terms, number of citations, author hindex, author number of publications and the Journal Similarity Factor) are computed using LDB. The Journal Impact Factor was obtained from the ISI Web of Knowledge website powered by Thomson Reuters. We have normalized the coefficient factors between 0 and 100 to obtain a coherent formula.
The paper references are ordered by the ranking function and presented in decreasing order of relevance to the user. Although it is impossible for a human to make an exhaustive reading of all the presented papers, the user can access and see all the papers returned in decreasing order of relevance. The tool presents 30 results per page.
We will now detail each of the terms that integrate the ranking function.
Number of MeSH terms
With this procedure we guarantee that the MEDLINE sample has papers that have a higher number of common MeSH terms with the sequence related papers, and is taken as the first step to a collection of relevant papers.
The table shows an example of four papers associated with the input sequences
Papers  MeSH1  MeSH2  MeSH3  MeSH4 

Paper1  0  1  1  1 
Paper2  1  1  1  1 
Paper3  0  1  1  0 
Paper4  1  0  1  1 
Total  2  3  4  3 
This way the papers that have more common MeSH terms associated to the sequences are valued.
Author’s number of publications
The number of publications of an author has also been considered in the formula. However it may happen that an author may have a large number of publications but with few citations, and these citations are in journals with a small impact factor, whilst another author may have a smaller number of publications with a high number of citations published in journals with high impact factors, which should be more relevant in the ranking formula. For each author we count the number of publications available. The authors with more than fifty publications get a number of publications of fifty. We believe that fifty is a good number of publications for a good author. The authors with a lower number of publications are not immediately discarded but are ranked lower. However the most common case is paper’s with more than one author. In this case, we consider the highest number of publications. As we do not disambiguate the author’s name, there is a slight chance that authors with the same name induce an error in the effective number of publications of a particular author.
Number of citations
A citation is a unique reference to an article, a book, a Web page, a technical report, a thesis or other published item. The number of citations of a paper has become a major indicator to evaluate scientific work. Although it has some drawbacks and it is not unanimously accepted by the scientific community, nevertheless we consider that it is one of the most important measures to estimate the impact of scientific published work in the scientific community. Citation indexes provide a means with which to measure the relative impact of articles in a collection of scientific literature [18]. The concept of citation indexing and searching was invented by Eugene Garfield [19] and anticipated the Science Citation Index. There are several citation indexing systems such as Google Scholar, CiteSeer and Scopus. These systems allow to search for a researcher’s number of citations however, we could not use them to obtain the number of citations of around 20 million MEDLINE publications due to PubMed service restriction policies.
We have computed, using LDB, the number of citations of MEDLINE scientific papers. Each sequence has a set of paper references associated, and each of these references has the bibliography associated. Most of the referenced papers are available in MEDLINE so we can obtain the number of citations of a paper cited by other MEDLINE papers inside MEDLINE.
The number of citations for a particular paper is shown to be more relevant and important in comparison with the number of publications. This is because an author may have a higher number of publications but that are not cited, whilst another author may have a smaller number of publications but highly cited. The impact of a piece of research is the degree to which it has been useful to other researchers [20]. However, the number of citations does not take into account the distribution of the several publications, e.g., a high number of citations but with very few highly cited scientific papers. However, we could not use them to obtain around 20 million citations for the MEDLINE publications due to PubMed service restriction policies.
The α value represents the devaluation coefficient in decreasing order of the paper’s scientific age
Age of Papers  α 

≤ 5 years  1.0 
≥ 5 years and ≤ 20 years  0.8 
≥ 20 years  0.6 
For example, the paper “The sequence of the human genome”, a well known paper in the Biological and Medical communities, has a score of 100 for the item “number of citations” in MEDLINE, and in Google it has 10240 citations (searched in 18.December.2012). As we are only considering counting in LDB, on one hand we have found less papers inside MEDLINE that cite the scientific paper “The sequence of the human genome” than in Google, besides BioTextRetriever devalues the number of citations by the paper scientific age, thus the number of citations in MEDLINE is much lower than the one found by Google.
hindex
Hirsch [21] proposed the hindex: “A scientist has an index h if h of his or her N papers have at least h citations each, and the other (N−h) papers have less than or equal to h citations each”. In other words, the hindex bases itself on publications ranked in descending order according to their number of citations. hindex is approximately proportional to the square root of the total citation counts [22].
The hindex is an index that attempts to measure both the productivity and impact of the published work of a researcher. It is based on the set of the scientist’s most cited papers and the number of citations that they have received in other publications. It combines both the number of papers and their quality (impact, or citations to these papers) [23]. The hindex is recognized by the ISI Web of Science by Thomson Reuters or Scopus by Elsevier, as an important indicator for assessing research impact [21, 24, 25].
Like the other bibliometric measures, hindex has advantages and limitations. Mathematically it is very simple to compute and it is easy to understand [21, 23].
Hirsch claims in [21] that the hindex performs better than other singlenumber criteria commonly used to evaluate the scientific output of a researcher (impact factor, total number of documents, total number of citations, citations per paper rate and number of highly cited papers).
For young researchers the hindex is not a very promising measure since they have few publications highly cited and thus will probably have a low hindex. One might say that the hindex favors the researchers that have many cited publications. A scientist with very few highly cited papers or a scientist with many lowly cited papers will have a weak hindex[24], [26]. To address this issue, Hirsch presented the “m parameter” in [21] that divides h by the scientific age of a scientist (number of years since the author’s first publication) to attenuate this problem.
Besides this, the hindex also depends on the database in use, reason which, alongside problems with common names and different spellings, makes its flaws very visible [27]. Hirsch [21] also refers this technical problem in obtaining the complete list of publications of scientists with very common names. To overcome this problem, the authors in [28] recommend that the hindex should be calculated with a list of publications authorized by the scientist and found in the Web of Science using a combination of the scientist’s name and address or affiliation.
The hindex should not be used to compare scientists from different disciplines [21]. The hindex does not take care of selfcitations which can increase a scientist’s hindex[29]. The hindex can also be used to measure the scientific output of institutions and research groups [30].
As was already mentioned, we have obtained using LDB, the number of citations of each paper’s reference inside MEDLINE. We collect and store for each paper author the number of publications. For each publication we count the MEDLINE internal number of references to that particular publication to obtain the number of citations.
Example of hindex computation for h =4 in this case
Rank of publications  1  2  3  4  5 

Number of citations  1988  8  7  6  4 
The first line indicates the order of each publication in ascendant order. The second line presents the number of citations in descending order. The author’s first publication has 1988 citations, the second publication has 8 citations, and so on. We know from the literature that the hindex of an author is h when the number of citations is equal or greater than the number of publications. A researcher has hindex h if, in the list of articles arranged in decreasing order of the number of citations of these articles, r=h is the highest rank such that the papers on rank 1, 2,..., h each have at least h citations [31]. Thus in the presented example the hindex is 4, because the author has four papers with more than four citations each.
As a paper may have more than one author (which is the most common case) we calculate the hindex for all the authors of a paper and select the highest hindex. If an author has a high hindex and the other authors have a smaller hindex, it means that at least one author is recognized by the scientific community as having prestige, and a prestigious author has valuable publications.
Journal impact factor
For the ranking we have considered only the journal Impact Factor because MEDLINE only references papers that are published in Journals. The Journal Impact Factor(JIF) [32] is a measure of the frequency with which the average article in a journal has been cited in a particular year or period, thus JIF may change overtime. JIF is based on information obtained from citation indexes. The most widely accepted and used JIF is from the Journal Citation Report (JCR), a product of Thomson Reuters ISI (Institute for Scientific Information) (only considers ISI journals). The Journal Citation Report has been published annually since 1975.
Garfield developed the journal’s impact factor metric that is defined by the following formula: $\mathit{\text{JIF}}=\frac{{C}_{2}}{{P}_{2}}$, where C_{2} is the number of citations in the current year of any of the items published in a journal in the previous 2 years and P_{2} is the number of papers published in the previous 2 years.
For example, the 2012 JIF is calculated by the formula: $\frac{{C}_{2}}{{P}_{2}}$, where: C_{2} is the number of times papers or other items published during 20102011 were cited in indexed journals during 2012, and P_{2} is the number of items published in 2010 plus 2011.
Thomson Reuters released in 2009, the new 5year journal Impact Factor in addition to the standard 2year journal Impact Factor. The 5year journal Impact Factor is the average number of times articles from a journal published in the past five years have been cited in Journal Citation Report year. And it is calculated by dividing the number of citations in the Journal Citation Report year by the total number of articles published in the five previous years.
The Journal Impact Factor is used to compare different journals only within the same field. The ISI Web of Knowledge indexes more than 11,000 science and social science journals.
A journal with a high impact factor is usually considered a high quality journal and high quality journals usually have high quality papers.
The Journal Impact Factor has some limitations stated by [33–35]:

Journal Impact Factor does not control selfcitations;

Journal Impact Factor varies significantly from field to field;

The Journal Impact factor depends on the dynamics of the research field;

Journals databases are not always accessible, i.e., neither all papers are available for free in the Web;

High citation rates do not always reflect the high quality of a journal/paper;

The Journal Impact Factor is calculated over a short period of time (the last two or five years);

A citation in a “low impact” journal is counted equally to a citation in a “high impact” journal, however they should be distinguished, since the second one it is more valuable than the first one;

A journal score is highly influenced by its total number of citable papers;

Journal Impact Factor does not assess the quality of individual papers (only a small percentage of Journal papers are highly cited but they have a huge impact in the total number of citations of a Journal).
[34, 35] also enumerate the determining factors associated with journals with high impact factors:

Indexing in most known databases:

PubMed/MEDLINE, Scopus and Google Scholar;

Papers written in the English language;

Availability of the fulltext paper, preferentially for free;

Availability of the paper abstract;

Submissions from authors with an higher reputation;

Publications of an higher number of review papers, because review papers are often more cited;

To cite papers previously published in the same journal;

Focus on dynamic “excellence” research fields that generate more citations.
Although the Journal Impact Factor has the above mentioned limitations we included it in the ranking formula.
We have obtained the 2year Journal Impact Factor for the papers that have been published in the Web of Knowledge website. We have downloaded the complete list of Journals Impact Factors available in October 2010^{h}. For each paper references BioTextRetriever retrieves, we gather the Journal Impact Factor of the publication (Thomson Reuters).
Journal similarity factor
Example of four publication journals of four papers associated to the input sequences
Papers  J1  J2  J3  J4 

Paper1  1  0  0  0 
Paper2  0  1  0  0 
Paper3  0  0  1  0 
Paper4  1  0  0  0 
Total  2  1  1  0 
Some of the items in the ranking formula, namely the number of MeSH terms in common with the papers associated with the sequences, the author’s hindex, the number of publications, the number of citations and the Journal Similarity Factor are calculated and stored jointly to the authors. The number of citations and the number of publications are independent of the other items. However the hindex relates the number of publications and the number of citations.
Choosing the ranking function coefficients
 1.
The number of MeSH terms associated with the papers connected to the sequences introduced by the user;
 2.
Number of PubMed publications;
 3.
Number of citations;
 4.
Author hindex;
 5.
Journal Impact Factor;
 6.
Journal Similarity Factor.
In order to assure the usefulness of these coefficients to the relevance of the retrieved papers and also to propose default values for the formula coefficients, we undertook a set of experiments that are next described.
Experimental settings
Data description
We have used 14 data sets, each one composed by more than 90 relevant papers. These data sets resulted from using sequences from 7 different domains with the following distribution:

Rnases: 1 sequence

Alzheimer: 1 sequence

Blood Pressure: 1 sequence

Erythrocites: 2 sequences

Hypertension: 2 sequences

Blood Glucose: 4 sequences

Lung Disease: 3 sequences
Characterization of data sets regarding the number of attributes and the number of positive and negative examples
Data sets  NA  Positive  Negative  Total 

examples  examples  examples  
S12  1602  128  128  156 
BP25  441  63  31  94 
ALZ31  1485  114  114  228 
ERYT11  1505  99  99  198 
ERYT21  1592  118  118  236 
HYP11  1706  130  130  260 
HYP21  1944  194  194  388 
BG11  1546  97  97  194 
BG21  1631  115  115  230 
BG31  1859  149  149  298 
BG41  1812  161  161  322 
LUNG11  1553  124  124  248 
LUNG21  1535  120  120  240 
LUNG31  1054  74  74  148 
Characterization of data sets used to tune the coefficients of the ranking function
Data sets  Total relevant papers  % Relevant papers 

classified by  classified by  
BioTextRetriever  BioTextRetriever  
S12  3947  78.9% 
BP25  2071  41.4% 
ALZ31  1751  35.0% 
ERYT11  2498  50.0% 
ERYT21  4397  87.9% 
HYP11  4235  84.7% 
HYP21  4638  92.8% 
BG11  4423  88.5% 
BG21  5  0.1% 
BG31  2301  46.0% 
BG41  3288  65.8% 
LUNG11  4103  82.1% 
LUNG21  4182  83.6% 
LUNG31  1644  32.9% 
In [14] we showed, empirically, that the best alternative was to use the Ensemble algorithms (Alternative 2) for the classification problem. Consequently we have used the results provided from this alternative in the experiments.
Experimental procedure
 1.
Run step 1 through step 5 of the tool to get a set of potentially relevant papers;
 2.
Add to the extracted set of papers classified as relevant in the previous step of the tool, 50 papers extracted randomly from the relevant papers associated to the input sequences. Since these papers are guaranteed to be relevant (by the owners of the original sequences) we use them to alternate the fact of not having access to an expert.
 3.
Count how many of the guaranteed relevant papers (obtained in 2.) will appear in high positions of the ranked set.
 4.
For each data set of the Table 7:
 (a)
Create 10 new sub data sets, each of them with 50 randomly examples added from the relevant one’s
 5.
The average of the combinations for each of the 10 sub data sets is obtained and represents the value achieved for each data set.
In these experiments we have tested the five coefficients with values from the set {0,25,50,75,100} with the restriction that the sum of all coefficients must be 100%. The combination of all these values for the five coefficients gives a total of one hundred and twenty six possible combinations.
The ranking function is evaluated by analyzing the first 20 papers that are presented to the user in descendant order of relevance and counting the number of papers from the 50 relevant ones inserted in the data set that appear in this 20 first.
The combination that has more hits in average for all the data sets is considered the best combination for the default ranking formula.
Results and discussion
The three best combinations for the fourteen data sets described in Table 7
Combination  C1  C2  C3  C4  C5  C6  Average 

comb1  0  75  25  0  0  0  3.8 (3.8) 
comb2  0  100  0  0  0  0  3.7 (4.1) 
comb3  0  50  50  0  0  0  3.6 (4.1) 
Data set  Comb1  Comb2  Comb3 

S12  1.9 (1.2)  1.9 (1.2)  1.6 (1.3) 
BP25  3.4 (1.2)  3.4 (1.2)  3.4 (1.2) 
ALZ31  3.1 (1.8)  3.3 (1.7)  2.8 (1.7) 
ERYT11  1.0 (1.2)  1.3 (1.2)  0.9 (0.9) 
ERYT21  4.1 (1.8)  3.8 (1.8)  3.7 (1.9) 
HYP11  0.9 (0.9)  0.9 (0.9)  0.7 (0.8) 
HYP21  1.9 (1.6)  1.7 (1.2)  1.8 (1.5) 
BG11  3.1 (1.3)  3.2 (1.7)  3.6 (1.4) 
BG21  17.6 (0.7)  16.4 (0.7)  17.5 (0.9) 
BG31  2.8 (1.7)  2.8 (1.7)  2.6 (1.6) 
BG41  2.0 (1.3)  2.1 (1.5)  2.0 (1.3) 
LUNG11  3.8 (2.2)  3.6 (1.9)  3.7 (2.2) 
LUNG21  2.7 (1.6)  2.8 (1.7)  2.1 (1.1) 
LUNG31  4.4 (1.2)  4.1 (0.9)  3.7 (0.9) 
The best results highlight the number of citations and the hindex factors. We have applied the ttest to analyze these three best results. The ttest (α=0.05) gave no statistical significance between the three best results presented.
From the presented best combinations, BioTextRetriever was configured with the combination presented in the first line of Table 9. Although BioTextRetriever was configured with the aforementioned weights, the user may introduce the weights.
Conclusions
We have developed a new methodology based on Machine Learning techniques to construct a classifier in real time for classifying MEDLINE papers. We have devised and assessed several ways of partitioning the data and combining the Machine Learning algorithms in order to achieve a good performance in the classification process. From this study we were able to conclude that the best Machine Learning algorithms to achieve a good performance are the Ensemble of Classifiers (a method that combines the individual decisions of a set of classifiers through majority or voting). In terms of the accuracy of the results, the Ensemble of algorithms achieved an accuracy of 95.3% and the stand alone classifiers achieved an accuracy of 92.7%. The results show that the use of Machine Learning is extremely valuable to automate the Information Retrieval process with good performance results.
In this paper we have proposed a new methodology that enables the automation of the assessment process of a multicriteria ranking function.
BioTextRetriever’s last procedure is to organize the papers selected as relevant by the classifier. In fact, this set of papers classified as relevant is quite large and it is not advisable to present such a huge number of papers to the user. We proposed an integrated ranking function that combines MeSH terms, PubMed number of citations, author PubMed hindex, journals impact factor, authors number of PubMed publications and journal similarity factor^{i}.
Since we do not have access to an expert to evaluate the results of the ranking function, we have adopted a procedure where the relevant papers associated to the original sequences are the ones that maximize the presented ranking function if they appear in the first 20 results. Since these papers are guaranteed to be relevant (because they are associated to the original sequences) we use them as an alternative to the fact that we do not have access to an expert. The ranking function is evaluated by analyzing the first 20 papers that are presented to the user in descendant order of relevance by the ranking function, and counting the number of papers from the relevant papers associated with the introduced sequences that maximize the ranking function. The best combinations maximize the number of citations and the hindex. BioTextRetriever was configured, by default, with this coefficients combination, however the user can introduce other weights for each factor.
Endnotes
^{a} We have used MEDLINE 2010.
^{b} The Medical Subject Headings (MeSH) [36] is a controlled vocabulary thesaurus maintained by the National Library of Medicine (NLM).
^{c} The journal similarity factor highlights the journals with more papers published associated with the original sequences.
^{d} evalue is a statistic to estimate the significance of a “match” between two sequences [37].
^{e} We have established that this would be 10% of the number of “not similar” sequences associated with the introduced sequence.
^{f} In all of the algorithms uses a wrapper was used to find the best algorithm’s parameter combination.
^{g} at most 5000.
^{h} At this date there were 7347 journal classifications available.
^{i} The journal similarity factor highlights the journals with more papers published that are associated to the original sequences.
Declarations
Authors’ Affiliations
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