Articles published in supplements in JCP are often cited when compared to the parent journal. The median number of citations from the 2000 cohorts appears to favor the supplements; although 5 years later there were no differences in the median number of citations in the 2005 cohorts. When adjusting for time by limiting the citation counts to the subsequent 3 calendar years after publication, the median number of citations for both the parent journal and the supplements were higher for the 2005 cohorts compared to the 2000 cohorts, but with a greater relative increase noted for the parent journal. The proportions of highly cited articles (articles that have generated at least 25 or 40 citations) were no different statistically between the parent journal and the supplements although a numerically higher proportion was consistently observed in favor of the parent journal for the 2005 cohort.
The mean number of citations per citable article shown in can be viewed as a “citability index” and is similar in concept to the journal impact factor 
. As with the journal impact factor, there is wide variability in citation rates for individual articles no matter the overall citability index 
The ISI database used in this study does not include all possible citation sources. For example, the Google Scholar database 
can yield higher numbers of citations 
. However it is doubtful that these additional sources would skew either towards citations to the parent journal or to its supplements enough to make a difference in the overall findings presented here.
A caveat is that the results of this bibliometric analysis of articles from JCP may not be generalizable to other journals that principally publish research reports and/or to those journals that do not regularly issue supplements. In general, multi-specialty and specialty journals can have diverse aims, scope, and editorial standards. Not all supplements from all journals are financed by pharmaceutical or medical device manufacturers. Some journals may already have in place a rigorous peer-review process for their supplements. This study should be replicated using another journal which makes much use of supplements, perhaps in a field of specialty other than psychiatry.
Citation counts by themselves may not be necessarily reflective of the impact an article has on actual day-to-day clinical practice; citations in the literature are done primarily by academics and not ordinarily by community practitioners. Moreover, the number of citations an article receives may not be representative of the number of readers who have accessed the article. An article may be clinically influential yet not often cited. Other metrics such as number of downloads from a website, or the number of reprints ordered by commercial interests, may provide additional information regarding how many persons have actually obtained a copy of the article (or supplement).
Whether or not citing articles from supplements is good practice is not addressed by this analysis. Citation rates may not be related to the quality of the cited articles. It is possible that the highly cited articles, regardless of where they were published, may contain information that other authors may have found useful to support their arguments. It is also possible that an article is cited to identify methodological shortcomings and to show how the citing study intends to rectify those limitations. A limitation of this study is that the individual quality of each cited report was not assessed, and no distinction was made between reports of randomized controlled trials, other types of original reports, and reviews. The contents of the supplements were not further scrutinized regarding funding source or potential biases in the articles themselves. There is some published work regarding quality of articles in journal supplements versus the parent publication, but all of it predates this decade 
. Also unexplored are whether articles published in supplements are preferentially cited by other articles that have been sponsored in some way.
Of additional interest is the impact of self-citation (i.e. citing one's previous publication in a new publication). Self-citation was reported in a bibliometric analysis of articles about diabetes mellitus in 170 clinical journals published in 2000 
. Nearly one-fifth of all citations were author self-citations 
. However in that report, original articles had twice the proportion of author self-citations compared with review articles. Assuming this observation is generalizable to the JCP, because supplements usually contain reviews and the parent journal usually contains a high proportion of research reports, the impact of author self-citation on differential citation rates between supplement and parent may be difficult to interpret. Another issue is that of journal self-citation, i.e. when publications in a journal cite previous publications in the same journal 
. Journal self-citation can have a positive effect on a journal's impact factor 
, and can potentially affect citation rates to articles in supplements and their parent journal.
Unaddressed in this analysis is the issue of ghost authorship 
and ghost management 
of supplements. “Ghost writing” was described in 1934 
and in that report the author recommended that the assistance of medical writers be acknowledged. Several decades later it is apparent that this acknowledgement is not always made. It may be that supplements may be fertile ground for this behavior, given the different degree of editorial scrutiny applied to supplements compared to their parent journals.
Articles published in JCP supplements are robustly cited, with many cited more often than many articles published in the parent journal. Thus articles in supplements may be quite influential in guiding clinical and research practice, as well as shaping critical thinking. However, articles in supplements are not subject to the same rigor of peer review as the parent journal and because they are printed under the sponsorship of commercial interests, they may be perceived as less than objective. A reasonable step to help improve this perception would be to ensure that supplements are peer-reviewed in the same way as regular articles in the parent journal.