Average H-Index depends on collection policy of the source.

Value for the H-index depends on source’s content and coverage.

WOS

• Citation data only for the records indexed by database.
• Limited number of journals.
• European bias.
• Calculates the H-index of an author from 1965 – current.
• Limited number of journals in non-English languages.
• Clear results from scholarly journals.
• “In press” articles are not indexed because of editorial policy.
• Only in their final form.
• Self-citations exclusion option.
• Access via subscription.

SCOPUS

Citation data only for the records indexed by 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.
• Limited number of journals in non-English languages.
• Clear results from scholarly journals.
• One version of a paper in results.
• “In press” articles are included.
• Self-citations exclusion option.
• Access via subscription.

GOOGLE SCHOLAR

• Scholar’s work 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 than in WOS and Scopus.
• Value of H-index may vary because of additional irrelevant citations results from non-scholarly citations (records from unknown sources and informal material).
• Not all of journals are of the same quality as those found in the Web of Science or Scopus.
• In some cases Scholar indexes preprint and journal version of a paper and provides in the results 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.
• Freely accessibly from the internet.

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 articles cited in the database, not just the indexed articles.

The optimal methodology to value H-index for researcher is to consult Google Scholar in addition to Web of Science or Scopus.

Different disciplines have different standards for citation.

It is unfair to compare H-index of researchers working in different disciplines:

• Discipline that uses more references per paper is expected to have higher index.
• Researchers in specialist subject may have lower h-index than those in more popular or currently-topical areas.
• Coverage in different disciplines is not the same in different sources.

Journals indexed in the WOS cover mainly the core disciplines in the natural sciences.

Journals indexed in the WOS and Scopus do not sufficiently cover fields such as computer science.

Journals indexed in the WOS do not sufficiently cover field of engineering science as well.

Google Scholar coverage in such science disciplines like Biology, Chemistry and Physics is less than in WOS or Scopus. Google Scholar coverage for the four Social science disciplines: Education, Economics, Sociology and Psychology as well as Computing is higher than in WOS and Scopus.

Number of Google Scholar citations for mathematicians and computer scientists higher than the WOS and Scopus, but lower for high-energy physicists.

Variations or errors in name spelling should be taken into consideration:

• Various spellings of an author’s name.
• Differently formatted names in one source.
• Alternative names for an author.
• Typographical errors.
• Combining different authors who share the same name.

The Author Identification provides a unique number for every author and helps find and recognize a specific author and to avoid confusion with other similarly named authors.

The most commonly used Author Identifiers are: Researcher ID (Thomson Reuters), Scopus Author Identifier (Scopus) and Online Researcher and Contributor ID.

Some tools (e.g., SCOPUS) have a filter to exclude self-citations, while others do not and require manual filtering.

Researchers who produce a small number of influential papers with high citation rates will have low h-index if citations to their other papers are low, even though their contribution to a discipline may be high.

Researchers with short publishing careers might appear disadvantaged by low h-indexes, even though their impact in a discipline may be high.