Ruggiero, V. R. 2012. Beyond Feelings: A Guide to Critical Thinking. Ninth Edition. New York: The McGraw-Hill Companies, Inc. (On line), (http://www.kwcps.k12.va.us/userfiles/270/Classes/4893/VRRuggieroBeyondFeelings9thedition.pdf). diakses 29 Januari 2015.