1 Division of Violence Prevention, National Center for Injury Prevention and Control, Centers for Disease Control and Prevention, Atlanta, GA.
2 Office of Communication Resources, National Center for Injury Prevention and Control, Centers for Disease Control and Prevention, Atlanta, GA.
3 Division of Emergency Medicine, University of Maryland, Baltimore, MD.
4 Division of Applied Public Health Training, Epidemiology Program Office, Centers for Disease Control and Prevention, Atlanta, GA.
Received for publication May 1, 2002; accepted for publication September 12, 2002.
![]() |
ABSTRACT |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
clinical medicine; epidemiology; public health; publishing
Abbreviations: Abbreviations: AIDS, acquired immunodeficiency syndrome; AJE, American Journal of Epidemiology; JCR, Journal Citation Reports.
![]() |
INTRODUCTION |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
One method that may be used to explore the interface between fields, including epidemiology and clinical medicine, is citation analysis. This method is commonly used in the information sciences to quantify the frequency with which defined user populations access information as it is circulated through the published literature. Citation analysis assesses the interaction between disciplines as represented by citation patterns (3). Put simply, citation analysis is a method for counting the number of times articles in a specific journal (i.e., index journal) cite (i.e., reference) articles published in journals that focus on different medical specialties. In addition, the number of times that articles from these journals cite the index journal can also be counted. Citation analysis attempts to answer the questions: "How often has a journal been cited? What journals have cited it? Is it the old or the newer material that is being cited?"
Although citation analysis can determine whether information has been exchanged between two fields, this methodology has limitations, even when used as an indirect measure of use of epidemiologic information by clinical specialties. For example, how and why information is cited is not revealed through these analyses; neither quality nor significance of usage can be measured; and citations may refer to full papers, technical communications, letters, and editorials, among others. Nonetheless, the process of systematically analyzing and mapping citation patterns within and between fields can shed light on whether a certain body of literature (e.g., epidemiology) is being used, via information transfer, by another area of science (e.g., clinical medicine).
This study uses citation analysis to examine citation activity between the epidemiologic literature and clinical science journals for the publication years 19831999. Specifically, we identify which clinical medical specialties use epidemiologic literature most and which specialties have only minimal use. We also examined citation trends over the 17-year period. This study builds upon an earlier citation analysis study that examined interactions and trends in the use of epidemiologic information by clinical medicine for the publication years 19761982 (2).
![]() |
MATERIALS AND METHODS |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Selection of journals
Given our interest in the exchange of information between epidemiology and clinical medicine, we chose the American Journal of Epidemiology (AJE) as our epidemiologic journal of reference. Selection of AJE facilitated comparison with the earlier citation analysis (2), because AJE was used as the index journal for that study as well. In previous and the current analyses, AJE was chosen because it had the highest citation ratio (e.g., citations per citable items published) for articles published in epidemiology and public health journals for the years spanning the study period (5).
To select the clinical medicine journals, we reviewed information from both the JCR cited and citing packages to compile a list of journals that cited information from AJE and a list of those that were cited by AJE. Although the information contained in the cited and citing packages is based on individual journal articles, the citation counts are attributed to the journal in which the article appears. Information provided in the JCR, therefore, summarizes data as journal citations rather than individual article citations. For each year of our 17-year study period, we identified the 100 journals that most often cited articles from AJE. We also determined the top 100 journals cited by articles published in AJE. After obtaining these lists for each individual year, we then combined them to create two lists (i.e., the "cited" and "citing" lists) for the entire 17-year period. Because many of the same journals accounted for high numbers of citations for each individual year, the combined lists for the entire study period contained fewer than 500 journals for each list. For this study, we chose to limit our more detailed analysis to the top 200 journals found on each of the combined lists. This cutoff captured approximately 60 percent of the citation data for each list (figure 1) and was similar to the proportion of citations used for the more detailed analysis in the earlier study (2).
|
Once journals were assigned to one of the 52 categories, we collapsed these groups into the following three general fields: clinical medical sciences (n = 28), basic science (n = 22), and epidemiology/public health (n = 2). Decisions about journal placement into one of these three general fields were reached by consensus of the study investigators and were based largely on journal content.
Measures and analysis
Our initial analysis included all citations from both lists, citations to AJE, and citations by AJE for the years 19831999. Citations were rank ordered by journal title for the 25 most frequently cited journals on both lists. The citations for the next 175 journal items were collapsed into a single category, and the remaining citations were collapsed into the "all other" category.
A more detailed analysis using only the information from the top 200 journals from each list (i.e., the "cited" and "citing" lists) was performed. For each clinical medical science category, the number of journal titles in the list of journals by subject categories in the JCR (6) was tabulated. We then tabulated the number and proportion of citations to AJE, by AJE, and combined (i.e., both the cited and the citing lists). We used these counts as very rough estimates of the interchange between epidemiologic literature and each of these specialties. Moreover, these counts were used to answer the questions, "Does a given medical specialty cite the epidemiologic literature as often as the converse?" and "Has the directionality of citations between various medical specialties and AJE changed since the previous citation analysis (2)?"
The number of citations between AJE and the three general categories of journals (i.e., clinical medical science, basic science, and epidemiology/public health) was tabulated for each year of the study. Trend lines representing the total combined citation activity between epidemiology and each of these three general areas were used to compare the extent of epidemiologic knowledge being exchanged across fields over time. Using the combined citation totals reveals the interactivity between epidemiology and the other fields and attempts to answer the question, "How much interchange is occurring between epidemiology and these general fields via the literature?" We also examined similar trends for the combined citation interaction between AJE and selected clinical specialty journals (i.e., general internal medicine, cancer, cardiovascular disease, infectious disease, and pediatrics) to compare exchange of information across these specialties.
To identify the most useful journals for delivering epidemiologic results to clinical medical professionals, we calculated the "epidemiologic impact factor" for the clinical journals that had the highest number of citations within AJE. The epidemiology impact factor for a journal takes into account two measures: 1) its proportion of all citations with AJE and 2) its "impact factor," the ratio between citations to articles in a journal and the number of citable articles published in that journal. The "impact factor" also suggests the relative importance of a journal in a given field (7). For example, a journal with an impact factor of 2.0 has its articles cited twice, on average, by other journals. In addition, the use of the impact factor eliminates some of the bias that favors journals by size and frequency (7), thereby making it possible to compare larger, more frequently published journals with small obscure journals. The epidemiologic impact factor for a journal was calculated as the product of its impact factor in 1999 (8) and the proportion of all citations with AJE attributable to that journal over the study period. We used this calculation to measure the relative importance of a journal for interchanging information with the epidemiologic literature.
![]() |
RESULTS |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
|
|
|
Time trends
Based on the citation patterns, interactions between AJE and all three general fields show steady increases from 1983 to 1999 (figure 2). The trend line for the clinical medical science journals is similar to that for epidemiology/public health journals. Combined citations for clinical medical science journals increased by 158 percent during the study period, from 3,009 in 1983 to 7,751 in 1999. For epidemiology/public health journals, the number of citations increased by 220 percent, from 1,793 in 1983 to 5,735 citations in 1999. For publications concerned with the field of basic science, the number of citations increased by 312 percent, from 502 to 2,066 during the same period.
|
|
|
![]() |
DISCUSSION |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
We found that four of the top five journals that cited articles in AJE were epidemiology/public health journals. This finding is also a reflection of journals supporting the development of their given field. We found that the journals most often cited by AJE were clinical medical science journals. Three of the top five were in the specialty category of general/internal medicine. A cancer journal ranked fifth. These findings suggest that the work being done in clinical medicine is highly relevant to epidemiologic research.
The frequency of citations, both by AJE and to AJE, among the three general fields has remained unchanged from the previous study (2). Clinical medical science continued to account for half of the total combined citations. However, our study demonstrated a greater balance in the direction of citations. In the earlier study, citings by AJE to clinical medical science journal articles represented 53 percent of all combined citations to other works, and citings by clinical medical science journals to AJE accounted for 43 percent of all journals that cited AJE in 19761982. In the present study, the citation rates for clinical medical literatures citing epidemiologic literature were equal to the citation rates for epidemiologic literatures citing clinical medical literature.
Overall, the medical specialties that demonstrated the highest combined citation rates with epidemiology over the entire 17-year study period have remained unchanged compared with the earlier study (2). Most fields identified by the previous study as having low rates also had low rates in our study. However, we found that certain specialties increased their interchange with the epidemiologic literature. Publications concerning endocrinology, which were found to have low citation rates with AJE (<1 percent) in the previous study (19761982), accounted for nearly 4.0 percent of total combined citations during 19831999. Geriatrics/gerontology, neuroscience, and ophthalmology journals were others showing increased citation rates with AJE since the early 1980s. Other specialty journals (e.g., those specializing in pediatrics and the respiratory system) increased only slightly or decreased in citation rates compared with the previous study period.
The infectious disease specialty category accounted for about 4 percent of total combined citations. Because the earlier study did not categorize infectious disease as a medical specialty (it was instead represented through the tropical medicine and dermatology/venereal diseases specialty categories), no cross-study comparisons can be made. In this study, we found an imbalance in citations between infectious disease journals and AJE (table 3). Infectious disease journals were more than twice as likely to cite articles from the epidemiologic literature (e.g., cited AJE) than the converse (e.g., citings by AJE). This pattern reinforces the fact that the epidemiologic information is highly relevant to infectious disease research.
Examination of trends showed upward slopes in combined citation rates among all three general fields over the study period, suggesting that the use of epidemiology has steadily increased across both basic and clinical sciences. Trends of selected clinical specialties suggest that cardiovascular and cancer specialties had the greatest growth in citation activity over the study period, whereas rates between infectious disease and epidemiology grew steadily only to level off in the late 1990s (figure 3).
The epidemiology impact quotient was used to identify individual clinical medical journals that have demonstrated high use of epidemiologic information. This measure reveals more about a journals level of contribution to the literature that links clinical medicine and epidemiology by accounting for factors such as size, frequency of publication, and circulation. Our results indicate that the best publishing venues for circulating epidemiologic findings and ideas that are relevant to clinical medicine might not be those journals that have the highest citation rates with AJE. Journals with the highest citation rates with AJE may not be as important as those with high citation rates and high journal impact factors. Adjusting for the epidemiology impact quotient, which takes both characteristics into account, resulted in substantial shifts in the rankings (table 4). The journals with the highest epidemiology impact quotient were in the general/internal medicine category and include the New England Journal of Medicine, the Journal of the American Medical Association, and The Lancet. Given their higher visibility and demonstrated support for epidemiology, these interdisciplinary journals are highly desirable venues for publishing epidemiologic research.
The improved balance in citation patterns compared with the previous study (2) illustrates an increased use of the epidemiologic literature by medical specialties. As epidemiology continues to become more useful and relevant to the field of clinical medicine, the fields become increasingly mutually dependent. This increasing interdependence has been described as creating a health climate in which "medical organizations and physicians economic viability is becoming dependent on population-based data, epidemiologic analysis, and public health strategies" (9, p. 36).
Shifts in citation patterns among specific specialties and epidemiology can be attributed to several factors. Shifting patterns could represent changes in the availability and accessibility of resources for research, which subsequently affect what and where research is published. The specialties that underutilized epidemiology, as well as those that show signs of decreasing use, may be accessing and publishing epidemiologic research within their own literature or within the general/internal medicine literature. In either case, in our analysis the use of epidemiologic research would be underestimated. In addition, the specialties that demonstrate low citation rates with the epidemiologic literature might not lend themselves to epidemiologic methods. For example, a hospital-based specialty (e.g., anesthesiology) may have less need for population-based data than an acute-care specialty (e.g., infectious disease), which has an established dependence on epidemiologic methods. Answers to these questions require in-depth study on the specialty level.
Examination of the number of articles indexed per publication year in Medline showed a 1.5-fold increase over the study period, from 287,777 in 1983 to 426,143 in 1999. The 2.5-fold increase in citation rates between clinical medical journals and AJE outpaced the growth of published materials, lending further evidence to the growing interdependence between the two fields. A sharp increase in the slopes in both the clinical medicine and epidemiology/public health citation curves beginning in 1993 could be attributed to several significant technical developments that occurred about this time. In the early 1990s, the Internet/World Wide Web became more prevalent in academic and research communities, thereby improving, increasing, and expediting access to published materials. In 1992, AJE switched from a monthly publication to a biweekly publication, potentially increasing the number of articles in circulation per publication year and subsequently increasing the number of expected citations to it (10). However, a Medline review of the number of indexed articles by AJE from 1992 to 1999 remained relatively constant. The increased frequency and improved access to AJE, coupled with the parallel upward sloping of the clinical medical science and the epidemiology/public health curves, suggest that the fields are growing in tandem. Epidemiology likely has been successful in identifying and meeting the needs of the clinical medical communities.
Shifts in citation patterns can be attributed to shifts in mortality trends and research priorities, as well as to the effective use of epidemiology in disease detection and prevention by infectious disease researchers. As the chronic disease-related health burden increased over time, so did the use of epidemiologic methods by chronic-disease researchers and the subsequent number of published epidemiologic studies addressing chronic-disease issues (11). The epidemiologic approach in application to chronic disease research continues to expand, as does the body of research linking many chronic diseases to infectious agents (12, 13).
Steady increases in citation rates to AJE by infectious disease journals from 1983 to 1995 may be attributed to the concurrent growth in acquired immunodeficiency syndrome (AIDS) research. AIDS-related journals (e.g., AIDS and the Journal of Acquired Immune Deficiency Syndrome) provide additional publishing opportunities for infectious disease researchers. These journals may be responsible for the dramatic decrease in the number of infectious disease-related articles published in AJE in the recent past (14). The leveling off in citation activity between epidemiology and infectious disease literature from 1996 through 1999 does not necessarily point to a slowing of the use of epidemiologic information among infectious disease researchers. The number of indexed articles by Medline for the journal AIDS, for example, has increased annually since its inception in 1988, indicating that AIDS researchers are seeking specific specialty journals in which to publish.
Our findings are subject to several limitations. The citation indexing system used to tabulate and analyze citation patterns does not comprehensively maintain citation information for literature from every country in the world. Journals indexed by the system are subject to inclusion criteria, which necessarily limit the pool of information from which results are drawn (15). For example, few non-English language and low circulation journals are indexed. The JCR, however, is the most comprehensive resource available, and the effect that these limitations have on our general findings is likely negligible.
Many variables can influence the impact factor of a journal and its ranking in journal lists. Title changes will have short-lived effects on the impact factor of a journal. The inclusion of review articles, which are generally cited more often than typical research articles, could result in a disproportionate number of citations, thereby elevating the title in rank and impact. The inclusion of letters could affect a journals rank and impact. However, most journals publish primarily substantive research or review articles, and statistical discrepancies are only rarely significant (16). Variations in the ranges of peak impact factor can occur between different disciplines; therefore, viewing journals in the context of their specific specialty field (e.g., cancer) may be more useful than viewing them within general fields (e.g., clinical medicine) (6). Notwithstanding these limitations, the journals with high impact factors are among the most prestigious and most selective (16, 17), which validates our use of the journal impact factor and citation counts in the construction of the epidemiology impact quotient.
Generalizing findings based on citation trends with a single journal is risky, because epidemiologic studies are published in a variety of journals. Although AJE has been identified as being the premier journal in the field and arguably the best platform for circulating epidemiologic research in and between fields, how and why information is cited are not revealed through these analyses. Citation analysis can measure neither the quality nor the significance of usage. High citation rates and high impact factors do not necessarily reveal the relevance or value of articles published in a particular journal. Nonetheless, the prestige garnered from a high profile journal with a widespread circulation is indisputable.
Our objective was to evaluate the exchange of epidemiologic information within the field of clinical medicine. The success of any information science is related to its relevancy and accessibility to a defined group of users. Citation analysis, in spite of its limitations, provides a useful means by which to measure these performance attributes. Steady upward trends in citation activity between epidemiology and the clinical medical sciences indicate that epidemiologists continue to generate highly relevant information products. These increases also suggest that epidemiologists are not merely becoming more prolific but are improving access to their work by publishing more frequently in journals with high impact factors. Apparent gaps and declines in the use of epidemiologic information could be interpreted in numerous ways but must be considered as opportunities for growth in the field of epidemiology. For example, the various clinical medical specialties identified in this study as having low citation activity with the epidemiologic literature (e.g., allergy, anesthesiology, and dermatology) may represent fields ripe for new epidemiologic studies. The results of this study suggest that, although epidemiologists appear to be effectively recognizing, responding to, and anticipating the information needs of the fields users working in clinical medicine, broadening their focus to include certain additional medical specialties may enhance their contributions.
![]() |
ACKNOWLEDGMENTS |
---|
![]() |
NOTES |
---|
![]() |
REFERENCES |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|