We collected data in November 2017 through searches via the PubMed interface along with calculations in Excel. Data were obtained and organized according to the year that records were created in PubMed to show the trend in items added to PubMed each year. Data were not organized by publication year because records are not always added to PubMed in the same year that they are published. PMC records were also retrieved by searching PubMed using the subset pubmed pmc[sb], which retrieves live or available journal article records in PMC (i.e., excluding articles currently under embargo).
Because PubMed provides a public access interface for MEDLINE, we investigated current practices relating to the addition of records in PubMed to examine whether MEDLINE-indexed articles continue to represent the majority of PubMed records. To do this, we obtained the number of new records added to PubMed in 1990 and to both PubMed and PMC for the years 2000 to 2017. We also obtained the number of new records that were indexed in MEDLINE and still in-process to be indexed in MEDLINE during the same time span in order to determine the ongoing percentage of records in each resource that is or will be indexed in MEDLINE.
For the purpose of our analysis, MEDLINE records included the records in PubMed marked as either medline or inprocess in the subset field. The subset field retrieves records by citation status, subject, or journal category, with the search tag [SB] [43 ]. The records marked as medline have been indexed with MeSH and, if relevant, may be linked to the NCBI Gene database or included in Supplementary Concept Records for substances that are not in MeSH [44 , 45 ]. The in-process records have been identified for inclusion in MEDLINE, but the indexing process is not completed yet; therefore, any records with the inprocess subset would be missed if a user searches PubMed using only MeSH (or any MEDLINE-only interface).
Data were collected on the number of author manuscript records in PMC from 2005 to 2017 along with the number of these records that were also indexed in MEDLINE to determine the percentage of PMC content that was author manuscripts and the percentage of author manuscripts that was indexed in MEDLINE in recent years. All trends were compared prior to and following the NIH public access mandate to determine whether the mandate served as a motivation for authors to deposit their articles and for journals to participate in PMC to attract potential authors’ submissions.
Using an adaptation of the Comparing Means and Proportions spreadsheet created by Princeton Data and Statistical Services, we performed descriptive analysis of trends via z-test across years, with a significance threshold of a=0.001. Supplementary calculations were made in Tableau Desktop 10.1.1 to create figures.Supplemental Appendix A provides the formulas we used to perform the statistical analyses.
We also interviewed Joyce E. B. Backus, NLM associate director for library operations; Kathryn Funk, NLM program manager for PMC; and Laurey Steinke, an expert consultant for the PMC journal review process and assistant professor at the University of Nebraska Medical Center Department of Biochemistry and Molecular Biology, for additional qualitative details regarding PubMed, MEDLINE, and PMC. Deborah Ozga, NLM head of the Index Section, and Rebecca Stanger, NLM journal publisher liaison, provided additional information via email.
Because PubMed provides a public access interface for MEDLINE, we investigated current practices relating to the addition of records in PubMed to examine whether MEDLINE-indexed articles continue to represent the majority of PubMed records. To do this, we obtained the number of new records added to PubMed in 1990 and to both PubMed and PMC for the years 2000 to 2017. We also obtained the number of new records that were indexed in MEDLINE and still in-process to be indexed in MEDLINE during the same time span in order to determine the ongoing percentage of records in each resource that is or will be indexed in MEDLINE.
For the purpose of our analysis, MEDLINE records included the records in PubMed marked as either medline or inprocess in the subset field. The subset field retrieves records by citation status, subject, or journal category, with the search tag [SB] [43 ]. The records marked as medline have been indexed with MeSH and, if relevant, may be linked to the NCBI Gene database or included in Supplementary Concept Records for substances that are not in MeSH [44 , 45 ]. The in-process records have been identified for inclusion in MEDLINE, but the indexing process is not completed yet; therefore, any records with the inprocess subset would be missed if a user searches PubMed using only MeSH (or any MEDLINE-only interface).
Data were collected on the number of author manuscript records in PMC from 2005 to 2017 along with the number of these records that were also indexed in MEDLINE to determine the percentage of PMC content that was author manuscripts and the percentage of author manuscripts that was indexed in MEDLINE in recent years. All trends were compared prior to and following the NIH public access mandate to determine whether the mandate served as a motivation for authors to deposit their articles and for journals to participate in PMC to attract potential authors’ submissions.
Using an adaptation of the Comparing Means and Proportions spreadsheet created by Princeton Data and Statistical Services, we performed descriptive analysis of trends via z-test across years, with a significance threshold of a=0.001. Supplementary calculations were made in Tableau Desktop 10.1.1 to create figures.
We also interviewed Joyce E. B. Backus, NLM associate director for library operations; Kathryn Funk, NLM program manager for PMC; and Laurey Steinke, an expert consultant for the PMC journal review process and assistant professor at the University of Nebraska Medical Center Department of Biochemistry and Molecular Biology, for additional qualitative details regarding PubMed, MEDLINE, and PMC. Deborah Ozga, NLM head of the Index Section, and Rebecca Stanger, NLM journal publisher liaison, provided additional information via email.