Bibliometrics

2007 Bibliometric Analysis for Papers on Topics Related to Endocrine Disrupting Chemicals (EDCs)
July 2007

This is a bibliometric analysis of the papers prepared by intramural and extramural researchers of the U.S. Environmental Protection Agencys (EPA) Endocrine Disrupting Chemicals (EDCs) Research Program. For this analysis, 519 papers were reviewed, and they were published from 1997 to 2007. These publications were cited 8,997 times in the journals covered by Thomsons Web of Science¹ and Scopus². Of these 519 publications, 455 (87.7%) have been cited at least once in a journal.

Searches of Thomson Scientifics Web of Science and Elseviers Scopus were conducted to obtain times cited data for the EDCs journal publications. The analysis was completed using Thomsons Essential Science Indicators (ESI) and Journal Citation Reports (JCR) as benchmarks. ESI provides access to a unique and comprehensive compilation of essential science performance statistics and science trends data derived from Thomsons databases. For this analysis, the ESI highly cited papers thresholds as well as the hot papers thresholds were used to assess the influence and impact of the EDCs papers. JCR is a recognized authority for evaluating journals. It presents quantifiable statistical data that provide a systematic, objective way to evaluate the worlds 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 EDCs papers are published are the Impact Factor and Immediacy Index. 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 journals relative importance, especially when compared to other journals in the same field. 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 report includes a summary of the results of the bibliometric analysis, an analysis of the 519 EDCs research papers analyzed by ESI field (e.g., Environment/Ecology, Pharmacology & Toxicology, Plant & Animal Science), an analysis of the journals in which the EDCs papers were published, a table of the highly cited researchers in the EDCs Research Program, and an indication of whether any patents have resulted from the program.

SUMMARY OF RESULTS

1. More than one-quarter of the EDCs publications are highly cited papers. 144 (27.8%) of the EDCs papers qualify as highly cited when using the ESI criteria for the top 10% of highly cited publications. This is 2.8 times the 10% of papers expected to be highly cited. 14 (2.7%) of the EDCs papers qualify as highly cited when using the ESI criteria for the top 1%, which is 2.7 times the number expected. 4 (0.8%) of these papers qualify as very highly cited when using the criteria for the top 0.1%, which is 8 times the number anticipated. 3 (0.6%) papers actually meet the 0.01% threshold for the most highly cited papers, which is 60 times the expected number for this program.
2. The EDCs papers are more highly cited than the average paper. Using the ESI average citation rates for papers published by field as the benchmark, in 11 of the 15 fields in which the 519 EPA EDCs papers were published, the ratio of actual to expected cites is greater than 1, indicating that the EDCs papers are more highly cited than the average papers in those fields. For all 15 fields combined, the ratio of total number of cites to the total number of expected cites (8,997 to 4,582.3) is 2.0, indicating that the EDCs papers are more highly cited than the average paper.
3. Nearly one-half of the EDCs papers are published in high impact journals. 213 of the 519 papers were published in the top 10% of journals ranked by JCR Impact Factor, representing 41.0% of EPAs EDCs papers. This number is 4.1 times higher than the expected 52 papers. 228 of the 519 papers appear in the top 10% of journals ranked by JCR Immediacy Index, representing 43.9% of EPAs EDCs papers. This number is 4.4 times higher than the expected 52 papers.
4. Four of the EDCs papers qualify as hot papers. Using the hot paper thresholds established by ESI as a benchmark, 4 hot papers, representing 0.8% of the EDCs papers, were identified in the analysis. Hot papers are papers that were highly cited shortly after they were published. The number of EDCs hot papers identified is 8 times higher than the expected 0.5 hot papers.
5. The authors of the EDCs papers cite themselves much less than the average author. 418 of the 8,997 cites are author self-cites. This 4.6% author self-citation rate is well below the accepted range of 10-30% author self-citation rate.
6. Seventeen of the authors of the EDCs papers are included in ISIHighlyCited.com, which is a database of the worlds most influential researchers who have made key contributions to science and technology during the period from 1981 to 1999.
7. No patents were issued and no patent applications were filed by investigators from 1997 to 2007 for research that was conducted under EPAs EDCs research program.

Highly Cited EDCs Publications

All of the journals covered by ESI are assigned a field, and to compensate for varying citation rates across scientific fields, different thresholds are applied to each field. Thresholds are set to select highly cited papers to be listed in ESI. Different thresholds are set for both field and year of publication. Setting different thresholds for each year allows comparable representation for older and younger papers for each field.

The 519 EDCs research papers reviewed for this analysis were published in journals that were assigned to 15 of the 22 ESI fields. The distribution of the papers among these 15 fields and the number of citations by field are presented in Table 1.

Table 1. EDCs Papers by ESI Fields

There are 144 (27.8% of the papers analyzed) highly cited EPA EDCs papers in 10 of the 15 fieldsAgricultural Sciences, Biology & Biochemistry, Chemistry, Clinical Medicine, Computer Science, Environment/Ecology, Multidisciplinary, Pharmacology & Toxicology, Plant & Animal Science, and Social Scienceswhen using the ESI criteria for the top 10% of papers. Table 2 shows the number of EPA papers in those 10 fields that meet the top 10% threshold in ESI. Fourteen (2.7%) of the papers analyzed qualify as highly cited when using the ESI criteria for the top 1% of papers. These papers cover 4 fieldsComputer Science, Environment/Ecology, Multidisciplinary, and Pharmacology & Toxicology. Table 3 shows the 14 papers by field that meet the top 1% threshold in ESI. The citations for these 14 papers are provided in Tables 4 through 7. There were 4 (0.8%) very highly cited EDCs papers in the fields of Environment/Ecology and Multidisciplinary. These papers, which met the top 0.1% threshold in ESI, are listed in Table 8. Three (0.6%) of the EDCs papers met the top 0.01% threshold in ESI, which is 60 times the expected number of papers that should meet this threshold for this size program. This paper is listed in Table 9.

Table 2. Number of Highly Cited EDCs Papers by Field (top 10%)

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

Table 4. Highly Cited EDCs Papers in the Field of Computer Science (top 1%)

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

Table 6. Highly Cited EDCs Papers in the Field of Multidisciplinary (top 1%)

Table 7. Highly Cited EDCs Papers in the Field of Pharmacology & Toxicology (top 1%)

Table 8. Very Highly Cited EDCs Papers (top 0.1%)

Table 9. Extremely Highly Cited EDCs Paper in the Field of
Multidisciplinary (top 0.01%)

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 11 of the 15 fields in which the EPA EDCs papers were published, the ratio of actual to expected cites is greater than 1, indicating that the EDCs papers are more highly cited than the average papers in those fields (see Table 10). For all 15 fields combined, the ratio of total number of cites to the total number of expected cites (8,997 to 4,582.3) is 2.0, indicating that the EDCs papers are more highly cited than the average paper.

Table 10. Ratio of Actual Cites to Expected Cites for EDCs 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 journals 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 EDCs papers published in the top 10% of journals, based on the JCR Impact Factor. Two hundred thirteen (213) of 519 papers were published in the top 10% of journals, representing 41.0% of EPAs EDCs papers. This indicates that nearly one-half of the EDCs papers are published in the highest quality journals as determined by the JCR Impact Factor, which is 4.1 times higher than the expected percentage.

Table 11. EDCs 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 EDCs papers published in the top 10% of journals, based on the JCR Immediacy Index. Two hundred twenty-eight (228) of the 519 papers appear in the top 10% of journals, representing 43.9% of the EDCs papers. This indicates that nearly one-half of the EDCs papers are published in the highest quality journals as determined by the JCR Immediacy Index, which is 4.4 times higher than the expected percentage.

Table 12. EDCs 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 shorterpapers 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., March-April 2007), but there were a number of hot papers identified from previous periods.

Using the hot paper thresholds established by ESI as a benchmark, 4 hot papers, representing 0.8% of the EDCs papers, were identified in two fields Environment/Ecology and Multidisciplinary. The number of EDCs hot papers is 8 times higher than expected. 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 EDCs papers. Of the 8,997 total cites, 418 are author self-citesa 4.6% 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 4.6% self-cite rate for the EDCs papers is well below the range for author self-citation.

Highly Cited Researchers

A search of Thomsons ISIHighlyCited.com revealed that 17 (1.6%) of the 1,096 authors of the EDCs papers are highly cited researchers. ISIHighlyCited.com is a database of the worlds 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 EDCs publications are presented in Table 14.

Table 14. Highly Cited Researchers Authoring EDCs Publications

Patents

No patents have been issued or patent applications filed by investigators from 1997 to 2007 for research that was conducted under EPAs EDCs research program.

¹ Thomson Scientifics Web of Science provides access to current and retrospective multidisciplinary information from approximately 8,830 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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