For each Work in OpenAlex, we have a field called referenced_works that is meant to list the works that were cited by that publication. The number of references included in OpenAlex for a particular work might be less than you observe in the PDF or even the Crossref record for that publication. There are a few reasons why that might be the case.
- for a work to be included in the referenced_works section, it must be a work that is in OpenAlex and so if one of the references isn't indexed in OpenAlex, we won't be able to include it in that section;
- sometimes the references included in the Crossref record for a publication differ from the reference section in the final PDF if references get added in production;
- often, references aren't included in the Crossref record for a publication-- in those situations, we try to get the references from the pdf directly, but parsing errors can happen and we can miss some of them;
- when we get reference information from Crossref or the PDF, we first try to match to OpenAlex works based on DOI (with high efficiency)-- when DOIs aren't included we try to match based on other metadata, but have less success in those situations.