Google Scholar, Microsoft Academic, Scopus, Dimensions, Web of Science, and OpenCitations' COCI: a multidisciplinary comparison of coverage via citations

New sources of citation data have recently become available, such as Microsoft Academic, Dimensions, and the OpenCitations Index of CrossRef open DOI-to-DOI citations (COCI). Although these have been compared to the Web of Science (WoS), Scopus, or Google Scholar, there is no systematic evidence of their differences across subject categories. In response, this paper investigates 3,073,351 citations found by these six data sources to 2,515 English-language highly-cited documents published in 2006 from 252 subject categories, expanding and updating the largest previous study. Google Scholar found 88% of all citations, many of which were not found by the other sources, and nearly all citations found by the remaining sources (89%-94%). A similar pattern held within most subject categories. Microsoft Academic is the second largest overall (60% of all citations), including 82% of Scopus citations and 86% of Web of Science citations. In most categories, Microsoft Academic found more citations than Scopus and WoS (182 and 223 subject categories, respectively), but had coverage gaps in some areas, such as Physics and some Humanities categories. After Scopus, Dimensions is fourth largest (54% of all citations), including 84% of Scopus citations and 88% of WoS citations. It found more citations than Scopus in 36 categories, more than WoS in 185, and displays some coverage gaps, especially in the Humanities. Following WoS, COCI is the smallest, with 28% of all citations. Google Scholar is still the most comprehensive source. In many subject categories Microsoft Academic and Dimensions are good alternatives to Scopus and WoS in terms of coverage.

💡 Research Summary

The article under review expands and updates the largest previous comparative study of citation coverage across multiple bibliographic databases. Using a sample of 2,515 highly‑cited English‑language documents published in 2006, the authors harvested a total of 3,073,351 citations from six sources: Google Scholar (GS), Microsoft Academic (MA), Scopus, Dimensions, Web of Science (WoS), and the OpenCitations Index of CrossRef open DOI‑to‑DOI citations (COCI). The analysis focuses on overall coverage, overlap between sources, and discipline‑specific performance across 252 subject categories.

Key findings are as follows. Google Scholar captured 88 % of all citations, far exceeding the other platforms. Most citations found by the other five databases (89 %–94 %) were also present in GS, while GS contributed a substantial number of unique citations not indexed elsewhere. Microsoft Academic was the second‑largest source, covering 60 % of the total citations. It included 82 % of Scopus citations and 86 % of WoS citations, and it outperformed Scopus in 182 subject categories and WoS in 223 categories. Nevertheless, MA showed notable gaps in physics, some engineering fields, and several humanities categories.

Dimensions ranked fourth overall, covering 54 % of citations. It overlapped with 84 % of Scopus citations and 88 % of WoS citations, and it surpassed WoS in 185 categories and Scopus in 36 categories. Its coverage gaps were most pronounced in the humanities. Scopus and WoS each covered roughly half of the total citations (54 % and 45 % respectively) and displayed a high degree of mutual overlap (89 %–94 %). COCI, the smallest source, accounted for only 28 % of citations, reflecting its reliance on openly available CrossRef DOI links.

Beyond raw coverage, the paper discusses practical considerations such as cost, accessibility, and API availability. Google Scholar is free but offers limited bulk‑download capabilities and variable metadata quality. Microsoft Academic provides a free API and broad coverage, yet its indexing of non‑English or non‑journal literature can be uneven. Dimensions operates under a freemium model, with free access for non‑commercial research, and benefits from integration with CrossRef. Scopus and WoS remain subscription‑based, offering curated, high‑quality metadata but at higher expense. COCI is fully open but constrained to citations that have been deposited in CrossRef.

The authors conclude that Google Scholar remains the most comprehensive citation source overall. However, for many disciplines, Microsoft Academic and Dimensions constitute viable, cost‑effective alternatives to Scopus and WoS, delivering comparable or even superior coverage. The study also highlights persistent discipline‑specific gaps—particularly in physics and certain humanities fields—where traditional subscription databases still hold an advantage. The paper recommends that researchers select databases based on a balance of coverage, cost, and functional features, and calls for greater standardisation and openness in citation data to support robust bibliometric analyses.