[ Note: The API calls below have been superceeded with the development of the Event Data project. See the latest API documentation for equivalent functionality Want to know how many times a Crossref DOI is cited by the Wikipedia? http://det.labs.crossref.org/works/doi/10.1371/journal.pone.0086859 Or how many times one has been mentioned in Europe PubMed Central?
Geoffrey Bilder (Mon,) studied this question.
Synapse has enriched 5 closely related papers on similar clinical questions. Consider them for comparative context: