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• 2006 Bibliometric Analysis of Papers on Topics Related to Global Change

Bibliometrics

2006 Bibliometric Analysis (Revised) for Papers on Topics Related to Global Change (GC) (papers Published 1998 to 2005)
March 2006 Report (Revised–June 2007)

This is a revised bibliometric analysis of the papers prepared by intramural and extramural researchers of the U.S. Environmental Protection Agency (EPA) on topics related to global change (GC).  This analysis was revised in June 2007 because the journals were initially categorized into the fields used by Thomson Scientific’s Essential Science Indicators (ESI) using information provided in Thomson’s Journal Citation Reports (JCR); for this revised analysis, the journals were categorized into ESI fields using the journal category list by ESI that is available on the Internet athttp://in-cites.com/journal-list/index.html. The Journal List for ESI was made available in 2006 and the current list contains all of the 12,734 journals covered for ESI up to December 31, 2006. This list is updated bimonthly by Thomson.  This revised bibliometric analysis will allow comparison of the results of this 2005 analysis to those of the analysis performed in 2007.

This is a bibliometric analysis of the papers prepared by intramural and extramural researchers of on topics related to global change.  For this analysis, 341 papers, reports, and books were reviewed, and they were published from 1998 to 2005. These publications were cited 3,694 times in the journals covered by Thomson’s Web of Science.¹ Of these 341 publications, 293 (86%) have been cited at least once in a journal.

Searches of Thomson’s Web of Science, Scopus² , and Google were conducted to obtain times cited data for the global change journal publications.  The analysis was completed using Thomson’s ESI and JCR as benchmarks. ESI provides access to a unique and comprehensive compilation of essential science performance statistics and science trends data derived from Thomson’s databases. For influence and impact measures, ESI employs both total citation counts by field and cites per paper scores. The former reveals gross influence while the latter shows weighted influence, also called impact.  JCR is a recognized authority for evaluating journals.  It presents quantifiable statistical data that provide a systematic, objective way to evaluate the world’s leading journals and their impact and influence in the global research community. The two key measures used in this analysis to assess the journals in which the EPA global change papers are published are the Immediacy Index and the Impact Factor.  The Immediacy Index is a measure of how quickly the “average article” in a journal is cited.  This index indicates how often articles published in a journal are cited within the same year and it is useful in comparing how quickly journals are cited.  The Impact Factor is a measure of the frequency with which the “average article” in a journal has been cited in a particular year.  The Impact Factor helps evaluate a journal’s relative importance, especially when compared to other journals in the same field.

The report is divided into three sections.  The first section presents an analysis of all 341 global change papers analyzed by ESI field (e.g., Environment/Ecology, Geosciences, Plant & Animal Science, Engineering). The second section presents an analysis of the global change papers by year of publication. The third section presents an analysis of the global change papers by focus area (e.g., air quality, regional assessment, ecosystems).  In each section, a summary of the results is provided as well as the data evaluated and a description of the analysis.

I.  Analysis of Global Change Publications by ESI Field

SUMMARY OF RESULTS

1. About one-fifth of the global change publications are highly cited papers. A review of the citations indicates that 67 (19.6%) of the global change papers qualify as highly cited when using the ESI criteria for the top 10% of highly cited publications. This is nearly twice the number expected for a typical program. Eight (2.3%) of the global change papers qualify as highly cited when using the criteria for the top 1%, which is more than twice the number expected. One (0.3%) of these papers qualifies as very highly cited (in the top 0.1%), which is 3 times the number expected. It is not surprising that no papers actually meet the 0.01% threshold for the most highly cited papers because the expected number for this program is 0.03 papers in the most highly cited category.
2. The global change papers are more highly cited than the average paper. Using the ESI average citation rates for papers published by field as the benchmark, in 9 of the 14 fields in which the EPA global change papers were published, the ratio of actual to expected cites is greater than 1, indicating that the global change papers are more highly cited than the average papers in those fields. For all 14 fields combined, the ratio of total number of cites to the total number of expected cites (3,694 to 2103.25) is 1.8, indicating that the global change papers are more highly cited than the average paper.
3. Nearly one-third of the global change papers are published in very high impact journals. Ninety-nine (99) of the 341 papers were published in the top 10% of journals ranked by JCR Impact Factor, representing 29% of EPA’s global change papers. This number is about 3 times higher than expected. One-hundred seven (107) of the 341 papers appear in the top 10% of journals ranked by JCR Immediacy Factor, representing 31% of EPA’s global change papers. This number is more than 3 times higher than expected.
4. Eight of the global change papers qualify as hot papers. Using the hot paper thresholds established by ESI as a benchmark, 8 hot papers, representing 2.4% of the global change papers, were identified in the analysis. Hot papers are papers that were highly cited shortly after they were published. The number of global change hot papers is 24 times higher than expected.
5. The authors of the global change papers cite themselves much less than the average author. Two-hundred fifteen (215) of the 3,694 cites are author self-cites. This 5.8% author self-citation rate is well below the accepted range of 10-30% author self-citation rate.
6. Twenty-seven of the authors of the global change papers are included in ISIHighlyCited.com, which is a database of the world’s most influential researchers who have made key contributions to science and technology during the period from 1981 to 1999.

Highly Cited Global Change Publications

The 341 global change papers reviewed for this analysis covered 14 of the 22 ESI fields.  The distribution of the papers among these 14 fields and the number of citations by field are presented in Table 1.

Table 1.  Global Change Papers by ESI Fields

There are 67 (19.65% of the papers analyzed) highly cited EPA global change papers in 7 of the 14 fields—Environment/Ecology, Geosciences, Multidisciplinary, Plant & Animal Science, Engineering, Clinical Medicine, and Economics & Business—when using the ESI criteria for the top 10% of papers.  Table 2 shows the number of EPA papers in those 7 fields that meet the top 10% threshold in ESI.  Eight (2.35%) of the papers analyzed qualify as highly cited when using the ESI criteria for the top 1% of papers.  These papers cover five fields—Multidisciplinary, Environment/Ecology, Geosciences, Plant & Animal Science, and Engineering.  Table 3 shows the 8 papers by field that meet the top 1% threshold in ESI.  The citations for these 10 papers are provided in Tables 4 through 8.  There was 1(0.3%) very highly cited global change paper in the Multidisciplinary field. This paper, which met the top 0.1% threshold in ESI, is listed in Table 9.  None of the global change papers actually met the top 0.01% threshold in ESI, which is to be expected.

Table 2. 

Number of Highly Cited Global Change Papers by Field (top 10%)

Table 3.  Number of Highly Cited Global Change Papers by Field (top 1%)

Table 4.  Highly Cited Global Change Papers in the Field of Multidisciplinary (top 1%)

Table 5.  Highly Cited Global Change Papers in the Field of
Environment/Ecology (top 1%)

Table 6.  Highly Cited Global Change Papers in the Field of Geosciences (top 1%)

Table 7. Highly Cited Global Change Paper in the Field of
Plant & Animal Science (top 1%)

Table 8.  Highly Cited Global Change Papers in the Field of Engineering (top 1%)

Table 9.  Very Highly Cited Global Change Paper in the
Field of Multidisciplinary (top 0.1%)

Ratio of Actual Cites to Expected Citation Rates

The expected citation rate is the average number of cites that a paper published in the same journal in the same year and of the same document type (article, review, editorial, etc.) has received from the year of publication to the present.  Using the ESI average citation rates for papers published by field as the benchmark, in 9 of the 14 fields in which the EPA global change papers were published, the ratio of actual to expected cites is greater than 1, indicating that the global change papers are more highly cited than the average papers in those fields (see Table 10).

Table 10.  Ratio of Actual Cites to Expected Cites for Global Change Papers by Field

JCR Benchmarks

Impact Factor.  The JCR Impact Factor is a well known metric in citation analysis.  It is a measure of the frequency with which the “average article” in a journal has been cited in a particular year.  The Impact Factor helps evaluate a journal’s relative importance, especially when compared to others in the same field.  The Impact Factor is calculated by dividing the number of citations in the current year to articles published in the 2 previous years by the total number of articles published in the 2 previous years. 

Table 11 indicates the number of global change papers published in the top 10% of journals, based on the JCR Impact Factor.  Ninety-nine (99) of 341 papers were published in the top 10% of journals, representing 29.0% of EPA’s global change papers. This indicates that nearly one-third of the global change papers are published in the highest quality journals as determined by the JCR impact factor, which is nearly 3 times the expected percentage.

Table 11.  Global Change Papers in Top 10% of Journals by JCR Impact Factor

Immediacy Index.  The JCR Immediacy Index is a measure of how quickly the average article in a journal is cited.  It indicates how often articles published in a journal are cited within the year they are published.  The Immediacy Index is calculated by dividing the number of citations to articles published in a given year by the number of articles published in that year.

Table 12 indicates the number of global change papers published in the top 10% of journals, based on the JCR Immediacy Index.  One-hundred seven (107) of the 341 papers appear in the top 10% of journals, representing 31.4% of the global change papers. This indicates that approximately one-third of the global change papers are published in the highest quality journals as determined by the JCR immediacy index, which is more than 3 times higher than the expected percentage.

Table 12.  Global Change Papers in Top 10% of Journals by JCR Immediacy Index

Hot Papers

ESI establishes citation thresholds for hot papers, which are selected from the highly cited papers in different fields, but the time frame for citing and cited papers is much shorter—papers must be cited within 2 years of publication and the citations must occur in a 2-month time period.  Papers are assigned to 2-month periods and thresholds are set for each period and field to select 0.1% of papers.  There were no hot papers identified for the current 2-month period (i.e., January-February 2006), but there were a number of hot papers identified from previous periods.

Using the hot paper thresholds established by ESI as a benchmark, 8 hot papers, representing 2.4% of the global change papers, were identified in five fields—Environment/Ecology, Engineering, Multidisciplinary, Plant & Animal Science, and Geosciences.  The hot papers are listed in Table 13.

Table 13.  Hot Papers Identified Using ESI Thresholds

Author Self-Citation

Self-citations are journal article references to articles from that same author (i.e., the first author).  Because higher author self-citation rates can inflate the number of citations, the author self-citation rate was calculated for the global change papers.  Of the 3,694 total cites, 215 are author self-cites—a 5.8% author self-citation rate.  Garfield and Sher³ found that authors working in research-based disciplines tend to cite themselves on the average of 20% of the time.  MacRoberts and MacRoberts⁴ claim that approximately 10% to 30% of all the citations listed fall into the category of author self-citation. Kovacic and Misak⁵ recently reported a 20% author self-citation rate for medical literature.  Therefore, the 5.8% self-cite rate for the global change papers is well below the range for author self-citation.

Highly Cited Researchers

A search of Thomson’s ISIHighlyCited.com revealed that 27 (3.5%) of the 773 authors of the global change papers are highly cited researchers.  ISIHighlyCited.com is a database of the world’s most influential researchers who have made key contributions to science and technology during the period from 1981 to 1999. The highly cited researchers identified during this analysis of the global change publications are presented in Table 14.

Table 14.  Highly Cited Researchers Authoring Global Change Publications

II.  Analysis of Global Change Publications by Year of Publication

SUMMARY OF RESULTS

1. About one-fifth of the global change publications are highly cited papers (this is twice the number expected for a typical program). From 1998 to 2005, the percentage of global change papers that qualified as highly cited when using the ESI criteria for the top 10% of highly cited publications ranges from a low of 7.4% to a high of 30.9%. From 2000 to 2003, there were 8 papers that qualified as very highly cited when using the ESI criteria for the top 1%, with the percentages ranging from 2.4% to 7.7%. These percentages are, on average, about 2.6 times higher than would be expected for a typical program. One paper published in 2003 meets the ESI criteria for the top 0.1%, which is about 3 times higher than would be expected for a typical program. None of the global change papers meet the ESI criteria for the most highly cited papers (top 0.01%). The expected number of papers in this top category for a program of this size would be 0.03 papers, so the absence of papers in this top category is not surprising.
2. The global papers are more highly cited than the average paper. Using the ESI average citation rates for papers published by field as the benchmark, the ratio of actual to expected cites is greater than 1 for all but one of the years analyzed (i.e.,2004). This indicates that the global change papers are more highly cited than the average papers published in these fields.
3. Nearly one-third of the global change papers are published in high impact journals as determined by the JCR Impact Factor of the journals in which the papers are published. For every year analyzed the number of global change papers published in high impact journals (the top 10% of journals) exceeds the expected 10% as determined by the JCR Impact Factor and Immediacy Index of the journals. The percentage of papers in high impact journals (by Impact Factor) ranges from 14.3% to 45.2%, which is 1.4 to 4.5 times higher than expected. The percentage of papers in high impact journals (by Immediacy Index) ranges from 14.3% to 38.1%, which is 1.4 to 3.8 times higher than expected.
4. The percentage of publications cited one or more times is consistently high from 1998 to 2003. The number declines as expected in the more recent years (i.e., 2004 and 2005) because most publications are not cited until a year or longer after they are published.
5. The authors of the global change papers cite themselves less than the average self-citation rate. The author self-citation rate for most years is well below the accepted range of 10-30% author self-citation rate. The only exception is in 2005, which is expected because most publications are not cited by other researchers until a year or longer after they are published.

6. There were hot papers published in 2000, 2002, 2003, and 2004. The absence of hot papers in 2005 is not surprising given the fact that the period during which the citations analyzed for determining hot paper status was incomplete.

This section of the report presents an analysis of the global change papers by year of publication from 1998 to 2005.  The data are presented by year in Table 15, which includes eight key bibliometric parameters.

The results of the analysis are presented below and the numbers link the findings with the corresponding data in Table 15.

1.   No. of Global Change Papers Analyzed—The number of global change publications has ranged from 27 to 56 per year over the period analyzed (i.e., 1998 to 2005).

2.   Total No. of Highly Cited Publications—ESI identifies four thresholds of highly cited papers—those in the top 10%, 1%, 0.1%, and 0.01%.  It is extraordinary for a publication to meet the threshold for the top 0.01%; these publications are rare and should not be expected in every program.  Using the ESI thresholds for the top 10% cited papers, about 20% of the global change publications are highly cited papers (this is twice the number expected for a typical program).  From 1998 to 2005, the percentage of global change papers that qualify as highly cited when using the ESI criteria for the top 10% of highly cited publications ranges from a low of 7.4% to a high of 30.9%.  From 2000 to 2003, there were 8 papers that qualify as very highly cited when using the ESI criteria for the top 1%, with the percentage ranging from 0% to 7.7%. These percentages are, on average, about 3,5 times higher than would be expected for a typical program.  One paper published in 2003 meets the ESI criteria for the top 0.1%, which is about 3 times higher than would be expected for a typical program. None of the global change papers meet the ESI criteria for the most highly cited papers (top 0.01%). The expected number of papers in this top category for a typical program of this size would be 0.03 papers, so the absence of papers in this top category is not surprising.

3.   Ratio of Actual to Expected Cites—The global papers are more highly cited than the average paper.  Using the ESI average citation rates for papers published by field as the benchmark, the ratio of actual to expected cites is greater than 1 for all but two of the years analyzed (i.e., 2004 and 2005).  This indicates that the global change papers are more highly cited than the average papers published in these fields.

4.   No. of Papers in High Impact Journals by Impact Factor—Nearly one-third of the global change papers are published in high impact journals as determined by the JCR Impact Factor of the journals in which the papers are published.  For every year analyzed, the number of global change papers published in high impact journals (the top 10% of journals) exceeds the expected 10% as determined by the JCR Impact Factor of the journals.  The percentage of papers in high impact journals (by Impact Factor) ranges from 14.3% to 45.2%, which is 1.4 to 4.5 times higher than expected.

5.   No. of Papers in High Impact Journals by Immediacy Index— Nearly one-third of the global change papers are published in high impact journals as determined by the JCR Immediacy Index of the journals in which the papers are published.  For every year analyzed, the number of global change papers published in high impact journals (the top 10% of journals) exceeds the expected 10% as determined by the JCR Immediacy Index of the journals. The percentage of papers in high impact journals (by Immediacy Index) ranges from 14.3% to 38.1%, which is 1.4 to 3.8 times higher than expected.

6.   Total No. of Publications Cited One or More Times—The percentage of publications cited one or more times is consistently high (88% or greater) from 1998 to 2003.  The number declines as expected in the more recent years (i.e., 2004 and 2005) because most publications are not cited until a year or longer after they are published.

7.   Total No. of Author Self Cites—The authors of the global change papers cite themselves less than the average self-citation rate.  The author self-citation rates in Table 15 are well below the accepted range of 10-30% author self-citation rate. The only exception is in 2005, which is expected because most publications are not cited by other researchers until a year or longer after they are published.

8.         No. of Hot Papers—Using the hot paper thresholds established by ESI as a benchmark, there were hot papers published in 2000, 2002, 2003, and 2004. Because only the top 0.1% of papers are selected as hot papers for each ESI field, the percentages of global change hot papers identified in Table 15 are 18 to 96 times higher than expected for these years.  The percentage of hot papers in 2004 and 2005 is probably low because this analysis was conducted before the 2 year period in which hot papers must be cited was completed.  ESI establishes citation thresholds for hot papers, which are selected from the highly cited papers in different fields, but the time frame for citing and cited papers is much shorter—papers must be cited within 2 years of publication and the citations must occur in a 2-month time period.  Papers are assigned to 2-month periods and thresholds are set for each period and field to select 0.1% of papers.  For papers published in 2004, the 2-year period of consideration would not conclude until 2006.  Similarly, for papers published in 2005, the 2-year period of consideration would not conclude until 2007.  The original bibliometric analysis was completed in March 2006; therefore, the number of hot papers among those published in 2004 and 2005 would be expected to be lower than if the analysis was completed in January 2008.

Table 15.  Key Bibliometric Parameters for Global Change Papers By Year (1998 to 2005)

III.  Analysis of Global Change Publications by Focus Area

This section of the report presents an analysis of the global change papers by focus area (i.e., air quality, ecosystems, human health, regional assessment, and water quality).  The data are presented by focus area in Table 16, which includes eight key bibliometric parameters. 

SUMMARY OF RESULTS

1. About one-fifth of the global change publications are highly cited papers (this is twice the number expected for a typical program). The percentage of global change papers that qualify as highly cited when using the ESI criteria for the top 10% of highly cited publications ranges from 2.4% for the air quality papers to 35.0% for the water quality papers. The water quality and human health focus areas have the highest percentages of highly cited publications when using the ESI criteria for the top 10%, and the number of highly cited papers in these areas is 3.5 to 2.9 times higher than expected. The ecosystems and water quality focus areas hold the lead positions when using the ESI criteria for the top 1% of papers, and the number of very highly cited papers in these areas is 2.9 to 2.5 times higher than expected. Only one paper in the ecosystems focus area meets the ESI criteria for the top 0.1% of papers, which is 6 times higher than the expected number for this focus area. None of the global change papers meet the ESI criteria for the most highly cited papers (top 0.01%), which is expected.
2. The global papers are more highly cited than the average paper. Using the ESI average citation rates for papers published by field as the benchmark, the ratio of actual to expected cites is greater than 1 for all but the air quality focus area. This indicates that the global change papers are more highly cited than the average papers published in these fields.
3. Nearly one-third of the global change papers are published in high impact journals as determined by the JCR Impact Factor and Immediacy Index of the journals in which the papers are published. For every year analyzed, the number of global change papers published in high impact journals (the top 10% of journals) exceeds the expected 10% as determined by the JCR Impact Factor and Immediacy Index of the journals. The percentage of papers in high impact journals (by Impact Factor) for air quality, ecosystems, and human health ranges from 21.4% to 75.6%, which is 2 to 8 times higher than expected. The percentage of water quality papers published in high impact journals (by Impact Factor) is the same as that expected (i.e., 10%). None of the regional assessment papers are published in high impact journals, but this could be attributed to the fact that this focus area had more non-journal publications than the other focus areas. The percentage of papers in high impact journals (by Immediacy Index) for all five focus areas is higher than expected, ranging from 11.4% to 65.8%, which is 1.1 to 7 times higher than expected.
4. In four of the five focus areas (i.e., ecosystems, human health, regional assessment, and water quality) the percentage of publications cited one or more times is very high (i.e., 88% to 95%). For the air quality focus area the percentage drops to approximately 60%, but this is attributed to high percentage of air quality papers published after 2003.
5. The authors of the global change papers cite themselves less than the average self-citation rate. The author self-citation rates range from 3.4% to 23.4%. The rates for the ecosystems, human health, regional assessment, and water quality focus areas are well below the accepted range of 10-30% author self-citation rate, and the rate for the air quality papers is 23.4%, which is within the accepted range.
6. There were hot papers published in each of the five focus areas. The highest percentage of hot papers (i.e., 9.1%) is in the regional assessment focus area, and the percentage of hot papers in the three of the other focus areas is consistent (i.e., approximately 2.4%). These percentages are 24 to 91 times higher than expected.

The results of the analysis are presented below and the numbers link the findings with the corresponding data in Table 16.

1.   No. of Global Change Papers Analyzed—More than half of the global change publications fall under the ecosystems focus area.  The number of global change publications for the remaining focus areas is approximately the same.

2.   Total No. of Highly Cited Publications—ESI identifies four thresholds of highly cited papers—those in the top 10%, top 1%, top 0.1%, and top 0.0.1%.  It is extraordinary for a publication to meet the threshold for the top 0.01%; these publications are rare and should not be expected in every program.  Using the ESI thresholds, about 20% of the global change publications are highly cited papers (this is 2 times the number expected for a typical program).  The percentage of global change papers that qualify as highly cited when using the ESI criteria for the top 10% of highly cited publications ranges from 2.4% for the air quality papers to 35.0% for the water quality papers.  The water quality and human health focus areas have the highest percentages of highly cited publications when using the ESI criteria for the top 10%, and the number of highly cited papers in these areas is 3.5 and 2.9 times higher than expected.  The ecosystems and water quality focus areas hold the lead positions when using the ESI criteria for the top 1% of papers, and the number of very highly cited papers in these areas is 2.9 and 2.5 times higher than expected.  Only one paper in the ecosystems focus area meets the ESI criteria for the top 0.1% of papers, which is 6 times higher than the expected number for this area. None of the global change papers meet the ESI criteria for the most highly cited papers (top 0.01%). The expected number of papers in this top category for a typical program of this size would be 0.03 papers, so the absence of papers in this category is not surprising.

3.   Ratio of Actual to Expected Cites—The global papers are more highly cited than the average paper.  Using the ESI average citation rates for papers published by field as the benchmark, the ratio of actual to expected cites is greater than 1 for all but one of the focus areas (i.e., air quality).  This indicates that the global change papers are more highly cited than the average papers published in these fields.

4.   No. of Papers in High Impact Journals by Impact Factor—Nearly one-third of the global change papers are published in high impact journals as determined by the Impact Factor of the journals in which the papers are published.  The percentage of papers in high impact journals for air quality, ecosystems, and human health ranges from 21.4% to 75.6%, which is 2 to 8 times higher than expected for these three focus areas.  The percentage of water quality papers published in high impact journals is the same as that expected (i.e., 10%).  None of the regional assessment papers are published in high impact journals, but this could be attributed to the fact that this focus area had more non-journal publications than the other focus areas.

5.   No. of Papers in High Impact Journals by Immediacy Index—Nearly one-third of the global change papers are published in high impact journals as determined by the Immediacy Index of the journals in which the papers are published.  The percentage of papers published in high impact journals ranges from 11.4% to 65.8%, which is 1.1 to 7 times higher than expected for these five focus areas. 
6.   Total No. of Publications Cited One or More Times—In four of the five focus areas (i.e., ecosystems, human health, regional assessment, and water quality) the percentage of publications cited one or more times is very high (i.e., 88% to 95%).  For the air quality focus area, the percentage drops to approximately 60%, but this can be attributed to the fact that 74% of the air quality papers are published after 2003 compared to 4% of the regional assessment papers.  In fact, 20% of the air quality papers were published in 2005, and only 6 of the 27 (22%) global change papers published in 2005 (all focus areas combined) have been cited one or more times.

7.   Total No. of Author Self Cites—The authors of the global change papers cite themselves less than the average self-citation rate.  The author self-citation rates range from 3.4% to 23.4%.  The rates for the ecosystems, human health, regional assessment, and water quality focus areas are well below the accepted range of 10-30% author self-citation rate, and the rate for the air quality papers is 23.4%, which is within the accepted range.

8.   No. of Hot Papers—Using the hot paper thresholds established by ESI as a benchmark, there were hot papers published in each of the five focus areas.  The highest percentage of hot papers (i.e., 9.1%) is in the regional assessment focus area, and the percentage of hot papers in three of the other focus areas is consistent (i.e., approximately 2.4%).  Because only the top 0.1% of papers are selected as hot papers for each ESI field, the percentages of global change hot papers identified in Table 16 are 24 to 91 times higher than expected.

Table 16.  Key Bibliometric Parameters for Global Change Papers by Focus Area

¹  Thomson’s Web of Science provides access to current and retrospective multidisciplinary information from approximately 8,700 of the most prestigious, high impact research journals in the world. Web of Science also provides cited reference searching.

² Scopus is a large abstract and citation database of research literature and quality Web sources designed to support the literature research process. Scopus offers access to 15,000 titles from 4,000 different publishers, more than 12,850 academic journals (including coverage of 535 Open Access journals, 750 conference proceedings, and 600 trade publications), 27 million abstracts, 245 million references, 200 million scientific Web pages, and 13 million patent records. 

³ Garfield E, Sher IH.  New factors in the evaluation of scientific literature through citation indexing.  American Documentation 1963;18(July):195-210.

⁴ MacRoberts MH, MacRoberts BR.  Problems of citation analysis: a critical review.  Journal of the American Society of Information Science 1989;40(5):342-349.

⁵ Kavaci N, Misak A.  Author self-citation in medical literature.  Canadian Medical Association Journal 2004;170(13):1929-1930.

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