Lorena Delgado-Quiros, Isidro F. Aguillo, Alberto Martín-Martín, Emilio Delgado Lopez-Cozar, Enrique Orduña-Malea, José Luis Ortega: Why are these publications missing? Uncovering the
reasons behind the exclusion of documents in free-access scholarly database, Journal of the Association for Information Science and Technology 2023 untersuchen, warum free Access Akademische Datenbanken wissenschaftliche Publikationen nicht vollständig erfassen: „The results show that coverage differences are mainly caused by the way each service builds their databases. While classic bibliographic databases ingest almost the exact same content from Crossref (Lens and Scilit miss 0.1% and 0.2% of the records, respectively), academic search engines present lower coverage (Google Scholar does not find: 9.8%, Semantic Scholar: 10%, and Microsoft Academic: 12%). Coverage differences are mainly attributed to external factors, such as web accessibility and robot exclusion policies (39.2%–46%), and internal requirements that exclude secondary content (6.5%–11.6%).“