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Hill, Elizabeth G.; Slate, Elizabeth H.; Wiegand, Ryan E.; Grossi, Sara G.; Salinas, Carlos F. . (2006). Study Design for Calibration of Clinical Examiners Measuring Periodontal Parameters. Journal of Periodontology, 77(7), 1129–1141. doi:10.1902/jop.2006.050395 
An interview with Sci-Hub Founder Alexandra Elbakyan,Who exactly should pay for academic research