At one time, only the Web of Science database collected citation count data. Originally, Science Citation Index, as it was known when first published in print in the early- to mid-1970s, focused on the article-centric science literature but now has expanded to almost every subject area. In addition, citation count data is found in many indexing or abstracting services. What was once a unique feature is now common place but therein lies some issues of concern.
Data contained in Experts.ASU is derived from Elsevier's Scopus database. As the name implies, only data for ASU faculty is available; for authors not in Experts.ASU, use the Scopus database to obtain a citation count. Because the data comes from the same source, you may intermix citation counts from Experts.ASU with citations counts from Scopus (provided the authors are in the same field and at the same career level.)
The Experts.ASU database has citation counts (derived from Scopus data) available for ASU faculty. Use the Scopus database for authors not in Experts.ASU or if you want to use different parameters than those selected in Experts.ASU.
The Scopus database covers some conference proceedings and books/book chapters in addition to journal articles and all these document types, unless you specify differently, will be in the results list. Originally Scopus only covered documents from 1996 to the present, however they are now updating pre-1996 cited references going back to 1970; consequently authors with works published from 1970-1995 may see more of their publications covered over time.
The citation count from the Web of Science database is calculated only from journal articles and does not count citations from conference papers, books or book chapters.
Google Scholar and other indexing and abstracting services such as SciFinder (chemistry) and PsycInfo (psychology) have citation counts for each article but they do not generate a citation report as Scopus and Web of Science does. Consequently, you'll have to add the citation counts for each article manually in order to obtain a total citation count for an individual.
Keep in mind:
Another option is Publish or Perish (PoP), free downloadable software that analyzes citation counts found through Google Scholar and Microsoft Academic Search. The software generates many different types of citation metrics useful for authors.