Krebs, EE, Carey, TS & Weinberger, M 2007, ‘Accuracy of the Pain Numeric Rating Scale as a Screening Test in Primary Care’, Journal of General Internal Medicine, Vol.22, no.10, hal. 1453–1458, diakses 27 Oktober 2013, http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2305860/