The number of citations depends on the content and coverage of the source.

WOS

• Citation data is available only for records that are indexed by the database
• A limited number of journals are included
• European bias
• Calculates the H-index of an author from 1965 – current
• A limited number of journals in non-English languages
• Clear results from scholarly journals
• Articles “in press” are not indexed due to editorial policy
• An option to exclude self-citations
• Access via subscription

SCOPUS

• Citation data is available only for records that are indexed by the database
• More content (about 22,000 journals) than in WOS (about 12,000 journals)
• American bias
• Calculates the H-index of an author from 1996 – current
• Scopus Cited References Expansion Program ensures cited references going back to 1970 will be added to pre-1996 Scopus content in the fourth quarter of 2014
• A limited number of journals in non-English languages
• Clear results from scholarly journals
• One version of a paper in results
• Articles “in press” are indexed
• An option to exclude self-citations
• Access via subscription

GOOGLE SCHOLAR

• Google Scholar’s works may be published in journals not covered by WOS and Scopus
• Larger number of journals than in WOS and Scopus *not all scholarly journals are indexed in Google Scholar
• Larger number of publications in non-English languages compared to both WOS and Scopus
• The value of the H-index may vary because of additional irrelevant citations results from non-scholarly citations (records from unknown sources and informal material)
• Not all the journals are of the same quality as those found in the Web of Science or Scopus
• In some cases, Google Scholar indexes  both the preprint and  the journal versions of a paper and provides the sum of the two counts, so citations are spread over the duplicates
• Provides an H-index only for researchers who created a user profile for themselves
• Counts self-citations
• Free access

CITESEERX

• CiteSeerX focuses primarily on the literature in computer and information science
• Uses ACI (Autonomous Citation Indexing) to automatically extract citations and create a citation index
• Computes citation statistics and related documents for all the articles that are cited in the database, not just the indexed articles

The optimal methodology to count citations is to consult Google Scholar and other sources in addition to Web of Science or Scopus.

• Disciplines that cite more references per paper are expected to drive up the Impact Factor of the journal
• The same journal may be ranked differently depending on the category being reviewed
• Different disciplines have different standards for citations.
  Fields like medicine and engineering typically have higher citation counts compared to humanities or social sciences.
  • Journals indexed in the WOS cover mainly the core disciplines in the natural sciences.
  • Journals included in WOS and Scopus offer limited coverage of disciplines like computer science and engineering.
  • Google Scholar’s coverage in disciplines such as biology, chemistry, and physics is lower compared to Web of Science (WOS) or Scopus. However, Google Scholar provides broader coverage for the four social science disciplines—education, economics, sociology, and psychology—as well as computing, compared to WOS and Scopus.
  • The number of Google Scholar citations for mathematicians and computer scientists is higher than that in WOS and Scopus, but lower for high-energy physicists.
• Coverage in different disciplines is not the same in different sources
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• Large and small journals are compared equally.
• Smaller or more niche journals will tend to have smaller Impact Factors.

Citation Manipulation refers to unethical practices that artificially inflate citation counts or influence metrics to enhance a researcher’s academic profile or career. These practices distort the true impact of research.

• Self-Citation
  Self-citation involves citing one’s own work to boost citation counts, misrepresenting a researcher’s impact. Some databases, like Scopus and Google Scholar, separate self-citations for clearer analysis.
• Citation Cartels
  Groups of researchers may agree to cite each other’s work to increase citations, distorting citation metrics.
• ‘Salami Slicing’
  This is the practice of breaking one study into several smaller papers to increase publication and citation counts without adding new content.
• Co-authorship Manipulation
  Some researchers may add excessive co-authors to papers to increase citations, even if their contributions are minimal.
• Publishing in Low-Quality Journals
  Publishing in low-quality journals can artificially inflate citation counts, even though the papers may not reach a broad scholarly audience.
• Plagiarism and “Recycling” Citations
  Researchers may reuse previous work or citations without adding new insights, artificially boosting citation metrics.
• Boosting Citations via Social Media
  Researchers may use social media or paid campaigns to promote their work, increasing its visibility and citation count.