Scientists cite only a fraction of prior studies

Scientists cite fewer than 25 percent of the relevant trials when writing about their own research. The more papers in the field, the smaller the fraction of previous papers that were quoted in a new study. Astonishingly, no matter how many trials had been done before in that area, half the time only two or fewer studies were cited. 

Samuel Arbesman, The Half-Life of Facts: Why Everything We Know Has an Expiration Date (New York: Current, Penguin USA, 2012), ch. 6 (citing Karen A. Robinson & Steven N. Goodman, "A Systematic Examination of the Citation of Prior Research in Reports of Randomized, Controlled Trials," Annals of Internal Medicine, vol. 154, no. 1, pp. 50-55 (Jan. 4, 2011))