In this bibliometric study, we present the results of publication on the topic of aromatherapy published between 1995 and 2014. The analyses of the growing trend of the number of original and review articles over the period, language used, and countries did not expose unexpected findings. Although over half of the 549 articles originated from authors from non-English speaking regions of the world, 95% of the articles were written in English. This finding reflects not only that English is the de facto global language of scientific communication [28] but also the characteristic of the Science Citation Index, which contains relatively few non-English language journals [29].
Of the 1888 authors, the two most prolific authors in aromatherapy revealed by this study, Edzard Ernst and Myeong Soo Lee were also highly productive in other subfields of complementary medicine. The distribution of the number of articles published by the authors in this study was evaluated with the Lotka’s law of scientific productivity [30]. Based on the statistical distribution of the productivity of authors based on Chemical Abstracts, Lotka observed that “… the number (of authors) making n contributions is about 1/n
2 of those making one; and the proportion all contributors, that make a single contribution, is about 60%.” In other words, the number of authors producing n articles is proportional to 1/n
2 or similarly, the number of journals containing n articles is proportional to 1/n
2. Our study found that only 12 authors contributed to five or more articles while 1654 of the 1888 authors (88%) contributed to only one article. However, the LOTKA computer program [24] failed to fit a Lotka’s power law distribution to our observed data.
On the other hand, the distribution of the 549 aromatherapy articles in the 287 journals fitted well, as indicated by the Kolmogorov–Smirnov goodness-of-fit test, according to the Lotka’s power law. A n of 2.424 and a c of 0.728 were obtained from the computer program LOTKA and therefore, the Lotka power function can be expressed as Y = 0.728/X2.424, where X is the number of articles and Y is the relative frequency of journals with X articles was obtained. According to this formula, 72.8% of the journals can be estimated to contain only one article. The value of n is larger than the 2 that originally suggested by Lotka but smaller than the 3.5 that recently reported in a study of citation data from the Scopus database [31]. Previous research indicated that the exponent n and the constant c could be influenced by the subject area and its productivity, the state of development, the country of origin, the time period of the study, and the length of that period [32].
As anticipated by the power law, most articles were concentrated in several journals. In fact, almost a quarter of the articles were published in only three journals, namely, Journal of Alternative and Complementary Medicine, Complementary Therapies in Medicine, and Evidence-based Complementary and Alternative Medicine. Hence, these journals can be considered as the core journals for knowledge dissemination of aromatherapy research.
The top 10 most cited original and review articles on aromatherapy published between 1995 and 2014 were analyzed to reveal the types of research in aromatherapy (Table 5). The top-ranking paper reported the findings from a postal survey of 2669 adults in England regarding their out-of-pocket expenditure on practitioner-provided complementary therapies and over-the-counter remedies [33]. The second-ranking paper was a review article published in 2007. The article described the possible mode of action of essential oils and their volatile constituents and outlined the therapeutic properties of essential oils in aroma and massage therapy [34]. Moreover, half of the 10 most cited articles were review articles. This finding is not surprising since systematic reviews and meta-analysis occupy the highest position in the current proposed hierarchy of evidence [35]. In addition, another possible reason for the high citation counts in review articles is because they are often cited under the introduction section of original articles. A study of the 100 most-cited papers in each of 21 scientific fields during the period 1996–2006 found that a considerable number of the articles in each field were review articles [36].
Visualization analysis was used to create a two-dimensional map of co-citation network of journals that received at least 50 co-citations (Fig. 2). Five clusters containing 55 journals were generated by the VOSviewer. The cluster 1 (red) formed by the 21 journals focusing on complementary medicine were distanced similarly from two other clusters: cluster 2 (green) formed by 15 journals in medicinal chemistry and food science and cluster 3 (blue) formed by 11 journals in general medicine and geriatric medicine. This distribution pattern indicated that while there was high relatedness among the articles within each cluster, moderate relatedness also existed between cluster 1 and 2 and between cluster 1 and 3. On the other hand, cluster 4 (purple) and especially cluster 5 (yellow) had much lower relatedness with the articles in the complementary medicine cluster.
To locate popular research topics on aromatherapy research published between 1995 and 2014, the co-occurrence of terms in the title or abstract of at least 20 articles was analyzed. Three clusters with a total of 50 terms were identified. Cluster 1 (red) consisted of terms related to essential oil such as the mode of administration (inhalation), type of oil (lavender), study design (experiment, placebo), and outcome (stress, blood pressure). The second cluster (green) composed of terms that dealt with interventions (aromatherapy, massage therapy) and medical conditions such as nausea, dementia, and cancer. Systematic reviews and reviews came out as prominent terms because 16% of the articles were reviews. In addition, the appearance of the term “child” reflected a few well-cited survey studies and reviews on complementary medicine use in children [37,38,39,40]. Finally, Cluster 3 (blue) contained general terms of complementary medicine, methods of knowledge acquisition (survey, questionnaire), and related complementary therapies (homeopathy, reflexology). The latter co-occurrence could be explained by the fact that these therapies were often included in survey studies [41,42,43] and disease-specific reviews [44, 45] of complementary medicine along with aromatherapy. The visualization of co-occurrence network could be used not only to show the pattern and hot spots of aromatherapy therapy in the past, but may also help to reveal potential or neglected research areas.
Several intrinsic limitations of this bibliometric analysis should be noted. First, it is possible that some articles could be missed with the use of a single citation database. Further studies can evaluate other databases such as Scopus and Google Scholars and to compare their findings with those from this study. Second, the Science Citation Index database is biased towards English-language journals and therefore, the results should be interpreted as such. Moreover, the non-English language journals included in the Science Citation Index database was found to have a lower impact than those in the English-language journals [29]. Therefore, the comparisons of publication output among countries might be affected.