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Lorenz, Karl A.; Sherbourne, Cathy D.; Shugarman, Lisa R.; Rubenstein, Lisa V.; Wen, Li; Cohen, Angela; Goebel, Joy R.; Hagenmeier, Emily et al. (1 May 2009). “How Reliable is Pain as the Fifth Vital Sign?”. J Am Board Fam Med22 (3): 291–298. doi:10.3122/jabfm.2009.03.080162. PMID19429735.