Explaining Citation Discrepancies Between Different Services

There are several citation providers in the industry including CrossRef, Web of Science, Scopus, and Google Scholar. They do not all index the same content:

• Crossref counts citations from data they receive from the many publishers who participate in the Crossref cited-by linking program. 
• Web of Science is very selective and only accepts certain journals into their database. Not everything published automatically gets indexed. 
• Scopus is also selective and will only index accepted content.
• Google Scholar indexes scholarly content from across the internet using an automated web crawler.

Additionally, these services will have different people writing different code/algorithms to retrieve citations, and they do not all use the same criteria to determine a true citation match.   Therefore we expect that there will be some discrepancy between the citation counts from these services.