Gella, Aleksander. Development of Class Structure in Eastern Europe: Poland and Her Southern Neighbors (en anglès). SUNY Press, 1988, p. 217. ISBN 9780887068331 [Consulta: 20 agost 2016]. «Oprichnina was originally a band of faithful servants organized by Ivan IV into a police force; they were used by the tsar to crush not only all boyars (Russian nobility) under suspicion, but also the Russian princes [...]. Oprichnina enabled the tsars to build the first police state in modem history.»
Wilson, Colin. Rasputin and the Fall of the Romanovs (en anglès). Citadel, 1964, p. 60. ISBN 9780806502809 [Consulta: 20 agost 2016]. «[Ivan IV] established a political security force to run the Oprichina[sic], whose task was to spy on his enemies and destroy them; hence Ivan may be regarded as the inventor of the police state.»
Casanova, Pablo González. Latin America Today (en anglès). United Nations University Press, 1 gener 1993, p. 233. ISBN 9789280808193 [Consulta: 22 març 2014].