As shown in Table 1, 68.3% and 76.9% of the publications in WoS and Scopus have a DOI match with Crossref. Focusing on research and review articles, these figures increase to 88.9% for WoS and 79.7% for Scopus. This demonstrates that a relatively large share of the publications not classified as research or review article in WoS lack a DOI. Matching based on DOIs involves various difficulties. Whe