PubMed as a Public Interface for MEDLINE: Assessing Whether PMC Has Changed PubMed’s Composition

Abstract

Objectives: Questions have arisen about the quality of PubMed due to PubMed Central (PMC) as a growing component of PubMed. We tested claims that content in PMC is of low quality and affecting PubMed’s reliability by evaluating the proportion of PubMed records indexed in MEDLINE over time and evaluating whether this aligns with the proportion of PMC records indexed in MEDLINE. Methods: The authors retrieved the following via the National Center for Biotechnology Information (NCBI) interface: the percentage of MEDLINE-indexed records added to PubMed in 1990 and in the years between and including 2000 and 2017, and the percentage of MEDLINE-indexed records added to PMC between 2000 and 2017. We performed a descriptive analysis of trends via z-test comparing the two resources and across all years of data collected. Results: Over 90% of PubMed records are indexed in MEDLINE; however, since the launch of PMC, the percentage of MEDLINE-indexed records added to PubMed each year has slowly decreased. This trend aligns with an increase in PMC content, which differs significantly from PubMed content in regards to MEDLINE composition. The largest impact comes from PMC full participation journals not indexed in MEDLINE, not from author manuscripts submitted to PMC in compliance with public access policies. Author manuscripts in PMC continue to be published in MEDLINE-indexed journals at a high rate (85%), but only comprise 2% of PubMed. Conclusions: The differing scopes of PMC and MEDLINE will likely continue to affect their overlap, and further emphasis by the National Library of Medicine on PubMed’s components will make PubMed users more clear about its scope. According to NLM, the aim is not to maintain a certain proportion of MEDLINE records in PubMed or PMC; the hope is that more MEDLINE-indexed journals will be deposited in PMC for long-term preservation and broader access. The authors also conclude that quality control exists in the maintenance and facilitation of both resources, but further research assessing article quality using critical appraisal methodology is needed.