How can I find the usage factor---the number of times a published scientific research article has been downloaded by readers?

This information is tracked by database vendors and is often kept private. Project Counter is an effort to set standards for reporting such download information and encouraging vendors to participate. 

"Usage factor" tracks the number of times that an article has been downloaded from a database. It is one of a number of what are called altmetrics, or metrics that can serve as complementary alternatives to the more traditional citation-based metrics such as a journal's impact factor. Citation Index/Web of Science includes information about impact factor, citation numbers for individual articles, and other metrics.

For further information on journal metrics of all kinds, see this helpful guide maintained by Johns Hopkins.

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