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Vanhecke TE, Barnes MA, Zimmerman J, Shoichet S (2007). "PubMed vs. HighWire Press: a head-to-head comparison of two medical literature search engines". Comput. Biol. Med. 37 (9): 1252–8. doi:10.1016/j.compbiomed.2006.11.012. PMID17184763.