When a journal article is indexed, numerous article parameters are extracted and stored as structured information.
Such parameters are: Article Type (Me SH terms, e.g., "Clinical Trial"), Secondary identifiers, (Me SH terms), Language, Country of the Journal or publication history (e-publication date, print journal publication date).
A reference which is judged particularly relevant can be marked and "related articles" can be identified.
If relevant, several studies can be selected and related articles to all of them can be generated (on Pub Med or any of the other NCBI Entrez databases) using the 'Find related data' option.
The secondary identifier field is to store accession numbers to various databases of molecular sequence data, gene expression or chemical compounds and clinical trial IDs.
For clinical trials, Pub Med extracts trial IDs for the two largest trial registries: Clinical (NCT identifier) and the International Standard Randomized Controlled Trial Number Register (IRCTN identifier).
Pub Med Central (PMC) is a free resource that provides access to millions of peer-reviewed articles in the biomedical and life sciences field, mainly from the MEDLINE database.
The archive is maintained by the US National Library of Medicine at the National Institutes of Health (NIH) and allows authors and publishers to store and cross-reference information from several sources using a common format within a single platform.
MEDLINE and Pub Med policies for the selection of journals for database inclusion are slightly different.
Weaknesses in the criteria and procedures for indexing journals in Pub Med Central may allow publications from predatory journals to leak into Pub Med.